lavc: remove reference to opt.h from Makefile.

Fixes make install after 3453a231a6.

libavcodec/Makefile

@@ -3,7 +3,7 @@ include $(SUBDIR)../config.mak
 NAME = avcodec
 FFLIBS = avutil
 
-HEADERS = avcodec.h avfft.h dxva2.h opt.h vaapi.h vdpau.h version.h xvmc.h
+HEADERS = avcodec.h avfft.h dxva2.h vaapi.h vdpau.h version.h xvmc.h
 
 OBJS = allcodecs.o                                                      \
        audioconvert.o                                                   \