libevent: mark util/monotonic_prc_fallback as retriable

Backport a patch to fix the below ptest failure:
 # ./run-ptest
 Running tests:
 EVPORT
 Skipping test
 KQUEUE
 Skipping test
 EPOLL
 test-eof: OKAY
 test-dumpevents: OKAY (output not checked)
 regress:
  FAIL ../libevent-2.1.12-stable/test/regress_util.c:1478: assert(diff.tv_sec == 0): 1 vs 0 util/monotonic_prc_fallback:
  [monotonic_prc_fallback FAILED]
 1/312 TESTS FAILED. (33 skipped)
 FAILED
 regress_debug:
  FAIL ../libevent-2.1.12-stable/test/regress_util.c:1478: assert(diff.tv_sec == 0): 1 vs 0 util/monotonic_prc_fallback:
  [monotonic_prc_fallback FAILED]
 1/312 TESTS FAILED. (33 skipped)
 FAILED
 [snip]

(From OE-Core rev: edecbbdacab5227c75e4a199e124389f8036b421)

[YOCTO #14507]

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>