build: Drop gcc-specific warning flag from header compilation rule

The flag was added to avoid excessive warning spam, but nowadays those
warnings no longer occur in such large numbers as to require silencing.
Besides, gcc-specific flags do not belong in the Makefiles.

Makefile

@@ -45,7 +45,7 @@ COMPILE_S = $(call COMPILE,AS)
 	$(COMPILE_S)
 
 %.ho: %.h
-	$(CC) $(CPPFLAGS) $(CFLAGS) -Wno-unused -c -o $@ -x c $<
+	$(CC) $(CPPFLAGS) $(CFLAGS) -c -o $@ -x c $<
 
 %.ver: %.v
 	$(Q)sed 's/$$MAJOR/$($(basename $(@F))_VERSION_MAJOR)/' $^ > $@