xen: Silence compiler warnings

Add inline keyword to silence the following compiler
warnings if xen_efi_probe() is not used:

  CC      arch/x86/xen/setup.o
In file included from arch/x86/xen/xen-ops.h:7:0,
                 from arch/x86/xen/setup.c:31:
include/xen/xen-ops.h:43:35: warning: ‘xen_efi_probe’ defined but not used [-Wunused-function]
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

include/xen/xen-ops.h

@@ -40,7 +40,7 @@ bool xen_running_on_version_or_later(unsigned int major, unsigned int minor);
 #ifdef CONFIG_XEN_EFI
 extern efi_system_table_t *xen_efi_probe(void);
 #else
-static efi_system_table_t __init *xen_efi_probe(void)
+static inline efi_system_table_t __init *xen_efi_probe(void)
 {
 	return NULL;
 }