vwebp_sdl: fix the makefile.unix
vwebp_sdl: fix the makefile.unix

The FLAGS_LIBS was dropped at some point.
There is still an SDL_Main / main problem though, but it's a
separate topic.

Change-Id: Ief4d79624e94e7904134df8d84a4ac7c77c48ce7

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