build/windows: Add comment inside *_sym.py script explaning its situation

While it is already explained on meson.build, it is desirable to
explain inside the script too in the case someone browse the files.

(cherry picked from commit 1730c62187)

build/windows/2_bundle-gimp-uni_sym.py

@@ -3,6 +3,9 @@ import os
 import shutil
 import fnmatch
 
+# This .py script should not even exist
+# Ideally meson should take care of it automatically.
+# See: https://github.com/mesonbuild/meson/issues/12977
 for build_root, _, build_bins in os.walk(os.getenv("MESON_BUILD_ROOT")):
   for file in build_bins:
     if fnmatch.fnmatch(file, '*.dll') or fnmatch.fnmatch(file, '*.exe'):