lib/test_debug_virtual.c: make struct pointer foo static

The pointer foo is local to the source and does not need to be
in global scope, so make it static.

Cleans up sparse warning:
symbol 'foo' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20180624112206.5722-1-colin.king@canonical.comSigned-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/test_debug_virtual.c

@@ -15,7 +15,7 @@ struct foo {
 	unsigned int bar;
 };
 
-struct foo *foo;
+static struct foo *foo;
 
 static int __init test_debug_virtual_init(void)
 {