x86/build: Supress realmode.bin is up to date message

Supress this unnecessary message during kernel re-build:

   make[3]: 'arch/x86/realmode/rm/realmode.bin' is up to date.

Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Link: http://lkml.kernel.org/r/1397584693-15902-1-git-send-email-pefoley2@pefoley.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>