tracing/xen: Hide events that are not used when X86_PAE is not defined

tracing/xen: Hide events that are not used when X86_PAE is not defined

TRACE_EVENTS() take up memory. If they are defined but not used, then
they simply waste space. If their use case is behind a define, then the
trace events should be as well.

The trace events xen_mmu_set_pte_atomic, xen_mmu_pte_clear, and
xen_mmu_pmd_clear are not used when CONFIG_X86_PAE is not defined.

Link: http://lkml.kernel.org/r/20171010191256.3d6d72cb@gandalf.local.home
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>