build: Properly remove object files while cleaning

Previously, object files in, for example, compat/ were left
after a clean or distclean was run.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

common.mak

@@ -52,4 +52,7 @@ CLEANSUFFIXES     = *.d *.o *~ *.h.c *.map *.ver
 DISTCLEANSUFFIXES = *.pc
 LIBSUFFIXES       = *.a *.lib *.so *.so.* *.dylib *.dll *.def *.dll.a
 
+clean::
+	$(RM) $(OBJS) $(OBJS:.o=.d)
+
 -include $(wildcard $(OBJS:.o=.d) $(HOSTOBJS:.o=.d) $(TESTOBJS:.o=.d) $(HOBJS:.o=.d))