x86/xen: Make xen_reservation_lock static

No users outside of that file.

Fixes: f030aade ("x86/xen: Move pv specific parts of arch/x86/xen/mmu.c to mmu_pv.c")
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel@lists.xenproject.org
Cc: boris.ostrovsky@oracle.com
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20180905053634.13648-1-jgross@suse.com

arch/x86/xen/mmu_pv.c

@@ -104,7 +104,7 @@ static pud_t level3_user_vsyscall[PTRS_PER_PUD] __page_aligned_bss;
  * Protects atomic reservation decrease/increase against concurrent increases.
  * Also protects non-atomic updates of current_pages and balloon lists.
  */
-DEFINE_SPINLOCK(xen_reservation_lock);
+static DEFINE_SPINLOCK(xen_reservation_lock);
 
 /*
  * Note about cr3 (pagetable base) values: