add missing rules for wmv2 compilation

Originally committed as revision 8968 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -181,6 +181,8 @@ OBJS-$(CONFIG_WMAV1_DECODER)           += wmadec.o wma.o
 OBJS-$(CONFIG_WMAV2_DECODER)           += wmadec.o wma.o
 OBJS-$(CONFIG_WMAV1_ENCODER)           += wmaenc.o wma.o
 OBJS-$(CONFIG_WMAV2_ENCODER)           += wmaenc.o wma.o
+OBJS-$(CONFIG_WMV2_DECODER)            += msmpeg4.o
+OBJS-$(CONFIG_WMV2_ENCODER)            += msmpeg4.o
 OBJS-$(CONFIG_WMV3_DECODER)            += vc1.o vc1data.o vc1dsp.o
 OBJS-$(CONFIG_WNV1_DECODER)            += wnv1.o
 OBJS-$(CONFIG_WS_SND1_DECODER)         += ws-snd1.o