gperf: depend on stamp file rather than gperf binary

There's no make rule to create the gperf binary, only the stamp file
so use that instead.

package/gperf/gperf.mk

@@ -20,7 +20,7 @@ GPERF_DEPENDENCIES = uclibc
 
 $(eval $(call AUTOTARGETS,package,gperf))
 
-$(GPERF_HOOK_POST_INSTALL): $(TARGET_DIR)/usr/bin/gperf
-	$(STRIPCMD) $(STRIP_STRIP_ALL) $^
+$(GPERF_HOOK_POST_INSTALL): $(GPERF_TARGET_INSTALL_TARGET)
+	$(STRIPCMD) $(STRIP_STRIP_ALL) $(TARGET_DIR)/usr/bin/gperf
 	touch $@