os: fix OpenBSD/PowerPC compilation

When attempting to re-enable the AltiVec support it was noticed
that we need to undefine _POSIX_C_SOURCE to appease the headers
for ff_get_cpu_flags_ppc() to be able to compile.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavutil/ppc/cpu.c

@@ -20,6 +20,7 @@
 #undef _POSIX_C_SOURCE
 #include <sys/sysctl.h>
 #elif defined(__OpenBSD__)
+#undef _POSIX_C_SOURCE
 #include <sys/param.h>
 #include <sys/sysctl.h>
 #include <machine/cpu.h>