unswitch: Fix up one unguarded fprintf (dump_file, ...) [PR123736]

This dump message is not guarded on dump_file being non-NULL, so crashes
inside of libc if dump_file is NULL.

I think a message like that is usually guarded not just on dump_file
being non-NULL, but also on TDF_DETAILS set in dump_flags.

2026-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/123736
	* tree-ssa-loop-unswitch.cc (hoist_guard): Guard dump message
	on dump_file && (dump_flags & TDF_DETAILS) condition.

(cherry picked from commit 613fef9239aa723ea65b1118262bb63d023169c4)