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>

drivers/platform/x86/dell-laptop.c

@@ -132,8 +132,8 @@ static struct dmi_system_id __devinitdata dell_blacklist[] = {
 };
 
 static struct calling_interface_buffer *buffer;
-struct page *bufferpage;
-DEFINE_MUTEX(buffer_mutex);
+static struct page *bufferpage;
+static DEFINE_MUTEX(buffer_mutex);
 
 static int hwswitch_state;