package/mpv: fixup fix build on powerpc64 w/ altivec

The initial fix for this issue, re-including stdbool.h, is not
working.

Instead, fix using the method used for other packages with a similar
problem: add -std=gnu99, which prevents bool from being redefined and
removes the need for any source code change.

Fixes: 64904f0f

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>