Makefile: pass $(CFLAGS) also during dependency generation

When Valgrind is not available NO_VALGRIND is set in CFLAGS, and this
is needed during dependency generation as well as compilation.
Message-Id: <20191210163033.9888-1-emaste@freefall.freebsd.org>

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -344,7 +344,7 @@ clean: libfdt_clean pylibfdt_clean tests_clean
 
 %.d: %.c
 	@$(VECHO) DEP $<
-	$(CC) $(CPPFLAGS) -MM -MG -MT "$*.o $@" $< > $@
+	$(CC) $(CPPFLAGS) $(CFLAGS) -MM -MG -MT "$*.o $@" $< > $@
 
 %.d: %.S
 	@$(VECHO) DEP $<