linux-yocto-rt: correct 3.0.3->3.0.4 mismerge for, stop_machine.c

linux-yocto-rt: correct 3.0.3->3.0.4 mismerge for, stop_machine.c

Merging the following commit for the -rt machines:

[
    preempt-rt/base: correct 3.0.3->3.0.4 mismerge for, stop_machine.c

    Commit 0b805cce57f61a244eb3b8fce460b14f1be442b3 dropped a change making
    stop_cpus_mutex non-static, resulting in a build failure for 3.0.4-rt
    kernels.

    Restore the move to non-static from commit
    6857336c7fddaf460a13adc0c395698fcf9423ff.

    Reported-by: Kishore Bodke <kishore.k.bodke@intel.com>
    (From OE-Core rev: 1bc26dca631da53d956008a4570c46012c478435)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
]

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>