Require libswscale only if the scale filter is used.

Although with several limitations, lavfi can be compiled and used
without the scale filter.

Originally committed as revision 24570 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/Makefile

 include $(SUBDIR)../config.mak
 
 NAME = avfilter
-FFLIBS = avcodec avcore avutil swscale
+FFLIBS = avcodec avcore avutil
+FFLIBS-$(CONFIG_SCALE_FILTER) += swscale
 
 HEADERS = avfilter.h