Log Message:
mpv: Pass `--bindir' to waf configure phase to fix installation with waf-2.0.2

With waf-2.0.2 update the mpv binary was tried to install apparently in:

 ${DESTDIR}${WRKSRC}/${PREFIX}/bin

while with previous waf versions it was correctly installed in

 ${DESTDIR}${PREFIX}/bin

Pass `--bindir' via WAF_CONFIGURE_ARGS to fix that problem.