xen: Fix 4.9.139 merge resolution

Commit 86e51430 ("Kbuild: use -fshort-wchar globally") applied this
flag globally so the issue noted in commit aadfde87 ("xen/efi: don't
use -fshort-wchar") is no longer valid. I have verified that the Pixel
3's kernel builds with this resolution.

Change-Id: I8fb92e81222e937070c4edb6b53b5f8b50fae5dd
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>