Fix crash with -D_GLIBCXX_DEBUG

Fix crash with -D_GLIBCXX_DEBUG

I noticed a buildbot failure where gdb crashed in info-os.exp, when
compiled with -D_GLIBCXX_DEBUG:

    (gdb) info os procgroups
    /usr/include/c++/7/bits/stl_algo.h:4834:
    Error: comparison doesn't meet irreflexive requirements, assert(!(a < a)).
    Objects involved in the operation:
	iterator::value_type "< operator type" {
	  type = pid_pgid_entry;
	}

The bug here is that pid_pgid_entry::operator< violates the C++
irreflexivity rule; that is, that an object cannot be less than
itself.

Tested locally by re-running info-os.exp.

gdb/ChangeLog
2018-07-30  Tom Tromey  <tom@tromey.com>

	* nat/linux-osdata.c (pid_pgid_entry::operator<): Fix
	irreflexivity violation.