x86: Fix mkcapflags.sh bash-ism

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

   http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html

Signed-off-by: Sylvain BERTRAND <sylvain.bertrand@gmail.com>
Link: http://lkml.kernel.org/r/20141223123912.GA1386@localhost.localdomain
Signed-off-by: Ingo Molnar <mingo@kernel.org>