Makefile: fix distclean target

It must imply clean. Regression since e0087a56.
Signed-off-by: James Almer <jamrial@gmail.com>

Makefile

@@ -138,7 +138,7 @@ clean::
 	$(RM) -r coverage-html
 	$(RM) -rf coverage.info coverage.info.in lcov
 
-distclean::
+distclean:: clean
 	$(RM) .version avversion.h config.asm config.h mapfile  \
 		ffbuild/.config ffbuild/config.* libavutil/avconfig.h \
 		version.h libavutil/ffversion.h libavcodec/codec_names.h \