testsuite: Fix up pr104601.C for recent libstdc++ changes

r14-9478 added [[nodiscard]] to various <algorithm> APIs including find_if
the pr104601.C testcase uses.  As it is an optimization bug fix testcase,
haven't tried to adjust the testcase to use the find_if result, but instead
have added -Wno-unused-result flag to quiet the warning.

2024-03-15  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/torture/pr104601.C: Add -Wno-unused-result to dg-options.