portmap: fix clean target to actually clean

Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/portmap/portmap.mk

@@ -19,6 +19,10 @@ define PORTMAP_BUILD_CMDS
 	$(MAKE) CC="$(TARGET_CC)" -C $(@D) $(PORTMAP_FLAGS)
 endef
 
+define PORTMAP_CLEAN_CMDS
+	$(MAKE) -C $(@D) clean
+endef
+
 define PORTMAP_INSTALL_TARGET_CMDS
 	for sbin in $(PORTMAP_SBINS); do \
 		$(INSTALL) -D $(@D)/$$sbin $(TARGET_DIR)/sbin/$$sbin; \