format_constants.h: MKFOURCC, correct cast
format_constants.h: MKFOURCC, correct cast

'd' should be promoted to uint32 before shifting by 24

Change-Id: I6212661af3802709b0098af8402ed73a0d9373ee

PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,webmproject/libwebp,Aexyn/libwebp,webmproject/libwebp,redengineer/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,Aexyn/libwebp,Aexyn/libwebp,PKRoma/libwebp,PKRoma/libwebp,redengineer/libwebp,Aexyn/libwebp,redengineer/libwebp,Aexyn/libwebp,ttyangf/libwebp,redengineer/libwebp,PKRoma/libwebp,ttyangf/libwebp,ttyangf/libwebp,redengineer/libwebp