package/libopenh264: fix mips32 build

Fix build failure with mips32 which is raised since the addition of
bootlin toolchains

Fixes:
 - http://autobuild.buildroot.org/results/cba3e9d0fd061cc3a92cb732bcdc2c7b66dbf6cb



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>