build: have `make clean` remove dep files too

While changing branches, one can hit errors like:

make[2]: ***   CC       libipt_CLUSTERIP.oo
No hay ninguna regla para construir el objetivo
`../include/net/netfilter/nf_nat.h', necesario para
`libipt_DNAT.oo'. Alto.

Pablo thinks dep files should be removed on `make clean`, and I
concur. (JFI, Note that native automake would not clear its ".deps"
directory.) Keep the "distclean: clean" line to keep invocations by
automake from the parent directory working.

Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/extensions/GNUmakefile.in b/extensions/GNUmakefile.in
index 218dc3a..4a8ff49 100644
--- a/extensions/GNUmakefile.in
+++ b/extensions/GNUmakefile.in
@@ -77,9 +77,9 @@
 
 clean:
 	rm -f *.o *.oo *.so *.a {matches,targets}[46].man initext.c initext4.c initext6.c;
+	rm -f .*.d .*.dd;
 
 distclean: clean
-	rm -f .*.d .*.dd;
 
 init%.o: init%.c
 	${AM_VERBOSE_CC} ${CC} ${AM_CPPFLAGS} ${AM_DEPFLAGS} ${AM_CFLAGS} -D_INIT=$*_init ${CFLAGS} -o $@ -c $<;