install dsputil.h, that is used by many filters in mplayer

Originally committed as revision 4825 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -515,6 +515,8 @@ install-headers:
 	mkdir -p "$(prefix)/include/ffmpeg"
 	install -m 644 $(SRC_PATH)/libavcodec/avcodec.h \
                 "$(prefix)/include/ffmpeg"
+	install -m 644 $(SRC_PATH)/libavcodec/dsputil.h \
+                "$(prefix)/include/ffmpeg"
 	install -d "$(libdir)/pkgconfig"
 	install -m 644 ../libavcodec.pc "$(libdir)/pkgconfig"