build: Fix compilation with ld.lld (#2094)

When using Clang 18<->22 with ld.lld, build fails with:
```
clang++ -Wl,--version-script=.../hidesymbols.map -o lib/liboslcomp.so.1.13.7 ...
ld.lld: error: version script assignment of 'global' to symbol 'test_shade' failed: symbol not defined
```

The issue is that test_shade does not belong to liboslcomp, instead it belongs to libtestshade,
and ld.lld actually checks that and fails. The minimal fix is to remove test_shade from hidesymbols.map,
the slightly better way is to create a separate symbols map for libtestshade.

See also: https://bugs.gentoo.org/929091

Signed-off-by: Sv. Lockal <lockalsash@gmail.com>

Author: AngryLoki

src/build-scripts/hidesymbols.map

@@ -1,4 +1,4 @@
 {
-    global: *OSL*; test_shade; *osl*imageio*; *osl_input_extensions; PyInit*;
+    global: *OSL*; *osl*imageio*; *osl_input_extensions; PyInit*;
     local: osl_*; *pvt*; *;
 };

src/testshade/CMakeLists.txt

@@ -110,6 +110,13 @@ if (NOT CODECOV)
                            OUTPUT_NAME libtestshade${OSL_LIBNAME_SUFFIX}
                            )
 
+    # Only libtestshade exports test_shade; keep it out of the global map.
+    if (VISIBILITY_MAP_COMMAND)
+        set_property (TARGET libtestshade
+                      APPEND PROPERTY LINK_FLAGS
+                      -Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/testshade_symbols.map)
+    endif ()
+
     target_link_libraries (libtestshade
                            PRIVATE
                                oslexec oslquery oslcomp)

src/testshade/testshade_symbols.map

@@ -0,0 +1,4 @@
+{
+    global: test_shade;
+    local: *;
+};