build: Remove redundant config.mak includes from subdirectory Makefiles.

Calling Make from subdirectories is not supported and config.mak has
multiple inclusion guards anyway, so the top-level include is enough.

libavcodec/Makefile

-include $(SUBDIR)../config.mak
-
 NAME = avcodec
 FFLIBS = avutil
 

libavdevice/Makefile

-include $(SUBDIR)../config.mak
-
 NAME    = avdevice
 FFLIBS  = avformat avcodec avutil
 

libavfilter/Makefile

-include $(SUBDIR)../config.mak
-
 NAME = avfilter
 FFLIBS = avutil
 FFLIBS-$(CONFIG_MOVIE_FILTER) += avformat avcodec

libavformat/Makefile

-include $(SUBDIR)../config.mak
-
 NAME = avformat
 FFLIBS = avcodec avutil
 

libavutil/Makefile

-include $(SUBDIR)../config.mak
-
 NAME = avutil
 
 HEADERS = adler32.h                                                     \

libpostproc/Makefile

-include $(SUBDIR)../config.mak
-
 NAME = postproc
 FFLIBS = avutil
 

libswscale/Makefile

-include $(SUBDIR)../config.mak
-
 NAME = swscale
 FFLIBS = avutil