OvmfPkg: add TimeBaseLib resolution for OvmfXen

OvmfPkg: add TimeBaseLib resolution for OvmfXen

Commit 55ee36b0c490
("EmbeddedPkg/RealTimeClockRuntimeDxe: Use helper functions from TimeBaseLib")
added a TimeBaseLib dependency to RealTimeClockRuntimeDxe, which now breaks
build of OvmfXen.dsc.

Add a resolution for EmbeddedPkg/Library/TimeBaseLib/TimeBaseLib.inf.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien@xen.org>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>