tracing/events: Fix swiotlb tracepoint creation

tracing/events: Fix swiotlb tracepoint creation

Tracepoints are only created when Xen support is enabled, but they are
also referenced within lib/swiotlb.c. So unless Xen support is enabled
the tracepoints will be missing, therefore causing builds to fail. Fix
this by moving the tracepoint creation to lib/swiotlb.c, which works
nicely because the Xen swiotlb support selects the generic swiotlb
support.

Signed-off-by: Thierry Reding <treding@nvidia.com>