[gobject] Drop const attribute from enum get_type declarations (#6061)

GLib removed G_GNUC_CONST from generated get_type declarations because
GType registration can initialize state and the attribute can cause
miscompilations with GCC trunk.

Match that behavior for generated GObject enum declarations.

Fixes: https://github.com/harfbuzz/harfbuzz/issues/6060

Assisted-by: OpenAI Codex

src/hb-gobject-enums.h.tmpl

diff --git a/src/hb-gobject-enums.h.tmpl b/src/hb-gobject-enums.h.tmpl
index 6dd98f7..3f491d9 100644
--- a/src/hb-gobject-enums.h.tmpl
+++ b/src/hb-gobject-enums.h.tmpl
@@ -43,7 +43,7 @@
 
 /*** BEGIN value-header ***/
 HB_EXTERN GType
-@enum_name@_get_type (void) G_GNUC_CONST;
+@enum_name@_get_type (void);
 #define @ENUMPREFIX@_TYPE_@ENUMSHORT@ (@enum_name@_get_type ())
 
 /*** END value-header ***/