package/libva: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 raised since bump to
version 2.20.0 in commit e926c83928e08c0ecd6cd4383e1cca151a8a4d2c and
https://github.com/intel/libva/commit/4f5a4345fc9f92565763ca22026891475e3cf835:

../va/va.c: In function 'va_new_opendriver':
../va/va.c:695:9: error: 'for' loop initial declarations are only allowed in C99 mode
         for (unsigned int i = 0; i < num_drivers; i++)
         ^
../va/va.c:695:9: note: use option -std=c99 or -std=gnu99 to compile your code

Fixes: e926c83928e08c0ecd6cd4383e1cca151a8a4d2c
 - http://autobuild.buildroot.org/results/b9dbd104fa05c59883d87f74e6522c55620a4252

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libva/libva.mk

@@ -10,6 +10,7 @@ LIBVA_LICENSE = MIT
 LIBVA_LICENSE_FILES = COPYING
 LIBVA_INSTALL_STAGING = YES
 LIBVA_DEPENDENCIES = host-pkgconf libdrm
+LIBVA_CFLAGS = $(TARGET_CFLAGS) -std=gnu99
 
 # libdrm is a hard-dependency
 LIBVA_CONF_OPTS = \