games/vvvvvv: Fix linking after sdl20 update

sdl20 was updated in r541153 and the cmake file was changed to add
directly /usr/local/lib/libSDL2 without adding -L/usr/local/lib/.
Since sdl2_mixer doesn't have a cmake file -L/usr/local/lib and this
port doesn't have other dependancy that adds it it fails to link.
Fix that by adding USES=localbase:ldflags

Reported by: pkg-fallout