[gimp] libgimpbase: avoid double definition of exported variables on Windows

[Ell <ell src gnome org>]

commit b2e223813bbcf862d0cce4c7a94a60a9d4e73e7a
Author: Ell <ell_se yahoo com>
Date:   Wed Jun 10 18:16:29 2020 +0300

    libgimpbase: avoid double definition of exported variables on Windows
    
    ... by qualifying them with "extern", in addition to
    "__declspec(dllexport)".  Omitting "extern" happened to work in the
    past, but recent GCC versions require it.

 libgimpbase/gimpenv.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/libgimpbase/gimpenv.h b/libgimpbase/gimpenv.h
index a9077be055..a6a26e7d3c 100644
--- a/libgimpbase/gimpenv.h
+++ b/libgimpbase/gimpenv.h
@@ -33,7 +33,7 @@ G_BEGIN_DECLS
 
 #ifdef G_OS_WIN32
 #  ifdef __GIMP_ENV_C__
-#    define GIMPVAR __declspec(dllexport)
+#    define GIMPVAR extern __declspec(dllexport)
 #  else  /* !__GIMP_ENV_C__ */
 #    define GIMPVAR extern __declspec(dllimport)
 #  endif /* !__GIMP_ENV_C__ */