avcodec: install avdct.h as public header

The commit 932ff709 introducing this header mentions it should be public.
Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/Makefile

@@ -3,6 +3,7 @@ include $(SUBDIR)../config.mak
 NAME = avcodec
 
 HEADERS = avcodec.h                                                     \
+          avdct.h                                                       \
           avfft.h                                                       \
           dv_profile.h                                                  \
           d3d11va.h                                                     \