Fix enable_tools=false breaking test builds

Move compat_cflags definition before the subdir_done() guard so the
variable is always available for test/meson.build regardless of the
enable_tools setting.

Author: kylophone

libvmaf/tools/meson.build

@@ -1,12 +1,12 @@
-if not get_option('enable_tools')
-    subdir_done()
-endif
-
 compat_cflags = []
 if cc.has_function('strsep')
   compat_cflags += '-DHAVE_STRSEP'
 endif
 
+if not get_option('enable_tools')
+    subdir_done()
+endif
+
 vmaf = executable(
     'vmaf',
     ['vmaf.c', 'cli_parse.c', 'y4m_input.c', 'vidinput.c', 'yuv_input.c'],