libmpeg2: enable ARM optimizations

(libmpeg2 does not autodetect on that architecture)

modules/codec/libmpeg2.c

@@ -211,6 +211,9 @@ static int OpenDecoder( vlc_object_t *p_this )
         i_accel |= MPEG2_ACCEL_PPC_ALTIVEC;
     }
 
+#elif defined(__arm__)
+    i_accel | MPEG2_ACCEL_ARM;
+
 #else
     /* If we do not know this CPU, trust libmpeg2's feature detection */
     i_accel = MPEG2_ACCEL_DETECT;