x86/purgatory, build: Suppress kexec-purgatory.c is up to date message

Suppress this unnecessary message during kernel re-build
(CONFIG_KEXEC_FILE=y):

make[1]: `arch/x86/purgatory/kexec-purgatory.c' is up to date.
Signed-off-by: NWANG Chao <chaowang@redhat.com>
Link: http://lkml.kernel.org/r/1413262019-3759-1-git-send-email-chaowang@redhat.comSigned-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/purgatory/Makefile

@@ -23,6 +23,7 @@ quiet_cmd_bin2c = BIN2C   $@
 
 $(obj)/kexec-purgatory.c: $(obj)/purgatory.ro FORCE
 	$(call if_changed,bin2c)
+	@:
 
 
 obj-$(CONFIG_KEXEC_FILE)	+= kexec-purgatory.o