arm_mpam: resctrl: Make resctrl_mon_ctx_waiters static

resctrl_mon_ctx_waiters is not used outside of this file, so make it
static. This fixes the sparse warning:

drivers/resctrl/mpam_resctrl.c:25:1: warning: symbol 'resctrl_mon_ctx_waiters' was not declared. Should it be static?

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603281842.c2K96tJA-lkp@intel.com/
Fixes: 2a3c79c ("arm_mpam: resctrl: Allow resctrl to allocate monitors")
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: James Morse <james.morse@arm.com>

Author: benhor01

drivers/resctrl/mpam_resctrl.c

@@ -22,7 +22,7 @@
 
 #include "mpam_internal.h"
 
-DECLARE_WAIT_QUEUE_HEAD(resctrl_mon_ctx_waiters);
+static DECLARE_WAIT_QUEUE_HEAD(resctrl_mon_ctx_waiters);
 
 /*
  * The classes we've picked to map to resctrl resources, wrapped