dell-laptop: Fix build error by making buffer_mutex static

The following build bug (x86, allyesconfig):

  arch/x86/oprofile/built-in.o:(.data+0x250): multiple definition of `buffer_mutex'

Was triggered in -tip testing, caused by this upstream commit:

  116ee77b: dell-laptop: Use buffer with 32-bit physical address

There's multiple buffer_mutex's in the kernel. Make this new one
static.

Signed-off-by: Ingo Molnar <mingo@elte.hu>