ffmpeg: fix build of shared libraries

009d8fceab4 and 0a5e5534cbf8 moved the --enable-static configure flag
from package files to the default _CONFIGURE_CMDS in
package/Makefile.autotools.in.
This broke Autotools packages that override _CONFIGURE_CMDS.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/multimedia/ffmpeg/ffmpeg.mk

@@ -167,6 +167,7 @@ define FFMPEG_CONFIGURE_CMDS
 		--arch=$(BR2_ARCH) \
 		--target-os=linux \
 		--extra-cflags=-fPIC \
+		$(SHARED_STATIC_LIBS_OPTS) \
 		$(FFMPEG_CONF_OPT) \
 	)
 endef