ffmpeg: fix altivec typo

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/multimedia/ffmpeg/ffmpeg.mk

@@ -150,7 +150,7 @@ endif
 # Set powerpc altivec appropriately
 ifeq ($(BR2_powerpc),y)
 ifeq ($(BR2_powerpc_7400)$(BR2_powerpc_7450)$(BR2_powerpc_970),y)
-FFMPEG_CONF_OPT -= --enable-altivec
+FFMPEG_CONF_OPT += --enable-altivec
 else
 FFMPEG_CONF_OPT += --disable-altivec
 endif