[gdb/testsuite] Fix kfail number in gdb.base/watchpoint-running.exp

Test-case gdb.base/watchpoint-running.exp reports the following kfail:
...
KFAIL: $exp: all-stop: software: watchpoint hit (timeout) (PRMS: gdb/111111)
...
but the referenced gdb PR doesn't exist.

Fix this by using an actual PR.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31833