Makefile.vc: define WEBP_HAVE_GIF for gifdec.c
Makefile.vc: define WEBP_HAVE_GIF for gifdec.c

Change-Id: Id0f93521e07689a7e40f6ed71446547aa576b83e

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,ttyangf/libwebp,ttyangf/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,ttyangf/libwebp