Fix return type typo in obsd-nat.c that breaks build on OpenBSD

Fix return type typo in obsd-nat.c that breaks build on OpenBSD

To recap the bug report:
Commit a068643 introduced a small typo that breaks the gdb build on OpenBSD.
Line 38 of obsd-nat.c needs to be changed from std::sring to std::string.

gdb/ChangeLog
2019-07-26  Brian Callahan  <bcallah@openbsd.org>

	PR gdb/24839:
	* gdb/obsd-nat.c (obsd_nat_target::pid_to_str): Fix typo in return
	type.