fix GCC15 compile error (#18737)

* fix GCC15 compile error

Author: ralfbrown

src/common/colorspaces.h

@@ -1,6 +1,6 @@
 /*
     This file is part of darktable,
-    Copyright (C) 2010-2024 darktable developers.
+    Copyright (C) 2010-2025 darktable developers.
 
     darktable is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -22,6 +22,8 @@
 
 #include <lcms2.h>
 
+G_BEGIN_DECLS
+
 // this was removed from lcms2 in 2.4
 #ifndef TYPE_XYZA_FLT
   #define TYPE_XYZA_FLT (FLOAT_SH(1)|COLORSPACE_SH(PT_XYZ)|EXTRA_SH(1)|CHANNELS_SH(3)|BYTES_SH(4))
@@ -419,6 +421,8 @@ void dt_make_transposed_matrices_from_primaries_and_whitepoint(const float prima
                                                                const float whitepoint[2],
                                                                dt_colormatrix_t RGB_to_XYZ_transposed);
 
+G_END_DECLS
+
 // clang-format off
 // modelines: These editor modelines have been set for all relevant files by tools/update_modelines.py
 // vim: shiftwidth=2 expandtab tabstop=2 cindent

src/imageio/format/imageio_format_api.h

@@ -1,6 +1,6 @@
 /*
     This file is part of darktable,
-    Copyright (C) 2016-2021 darktable developers.
+    Copyright (C) 2016-2025 darktable developers.
 
     darktable is free software: you can redistribute it and/or modify
     it under the terms of the GNU Lesser General Public License as published by
@@ -20,8 +20,6 @@
 
 #ifdef FULL_API_H
 
-G_BEGIN_DECLS
-
 #include <stddef.h>
 #include <stdint.h>
 
@@ -31,6 +29,8 @@ struct dt_dev_pixelpipe_t;
 
 #include "common/colorspaces.h" // because forward declaring enums doesn't work in C++ :(
 
+G_BEGIN_DECLS
+
 /* early definition of modules to do type checking */
 
 #pragma GCC visibility push(default)

src/lua/lua.h

@@ -1,6 +1,6 @@
 /*
    This file is part of darktable,
-   Copyright (C) 2013-2024 darktable developers.
+   Copyright (C) 2013-2025 darktable developers.
 
    darktable is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -28,6 +28,11 @@
 #include <glib.h>
 
 #ifdef USE_LUA
+
+// while it's poor practice to have includes inside an "extern C" block, we need to
+// do this here because the Lua headers don't supply it when compiling under C++....
+G_BEGIN_DECLS
+
 #include <lautoc.h>
 #include <lauxlib.h>
 #include <lua.h>
@@ -97,6 +102,8 @@ typedef struct
 } dt_lua_state_t;
 #endif
 
+G_END_DECLS
+
 // clang-format off
 // modelines: These editor modelines have been set for all relevant files by tools/update_modelines.py
 // vim: shiftwidth=2 expandtab tabstop=2 cindent