x86/viridian: make build_assertions static

This is consistent with other instances of the same function
and also resolves a violation of MISRA C:2012 Rule 8.4.

No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Paul Durrant <paul@xen.org>

xen/arch/x86/hvm/viridian/synic.c

@@ -18,7 +18,7 @@
 #include "private.h"
 
 
-void __init __maybe_unused build_assertions(void)
+static void __init __maybe_unused build_assertions(void)
 {
     BUILD_BUG_ON(sizeof(struct hv_message) != HV_MESSAGE_SIZE);
 }