avutil/cpu: add missing check for mmxext to av_force_cpu_flags

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/cpu.c

@@ -49,6 +49,7 @@ static int flags, checked;
 void av_force_cpu_flags(int arg){
     if (   (arg & ( AV_CPU_FLAG_3DNOW    |
                     AV_CPU_FLAG_3DNOWEXT |
+                    AV_CPU_FLAG_MMXEXT   |
                     AV_CPU_FLAG_SSE      |
                     AV_CPU_FLAG_SSE2     |
                     AV_CPU_FLAG_SSE2SLOW |