x86/build: Fix mkcapflags.sh bash-ism

Chocked while compiling linux with dash shell instead of bash
shell. See:

  http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_09_05



Signed-off-by: Sylvain BERTRAND <sylvain.bertrand@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20141229154324.GA27533@dhcppc1


Signed-off-by: Ingo Molnar <mingo@kernel.org>