vm_event: fix MISRA C 2012 Rule 8.7 violation

vm_event: fix MISRA C 2012 Rule 8.7 violation

The function vm_event_wake() is referenced only in vm_event.c.
Change the linkage of the function from external to internal by adding
the storage-class specifier static to the function definition.

Also, this patch aims to resolve indirectly a MISRA C 2012 Rule 8.4 violation
warning.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>