Makefile: fix distclean target

It must imply clean. Regression since 7ebe7e8e7a.

Signed-off-by: James Almer <jamrial@gmail.com>

Makefile

@@ -171,7 +171,7 @@ clean::
 	$(RM) $(addprefix compat/,$(CLEANSUFFIXES)) $(addprefix compat/*/,$(CLEANSUFFIXES))
 	$(RM) -rf coverage.info lcov
 
-distclean::
+distclean: clean
 	$(RM) .version avversion.h config.asm config.h mapfile \
             avbuild/.config avbuild/config.* libavutil/avconfig.h \
             libavcodec/bsf_list.c libavformat/protocol_list.c