Fix build failure in linux-osdata.c

Fix build failure in linux-osdata.c

Commit

  Use std::vector in linux_xfer_osdata_processgroups
  b129dcac882da87767cb863ad8b2572f90e9e5bc

broke the build with older gcc (at least 4.7 and 4.8):

In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from /home/emaisin/src/binutils-gdb/gdb/nat/linux-osdata.c:40:
/usr/include/c++/4.7/bits/stl_algo.h: In instantiation of ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pid_pgid_entry*, std::vector<pid_pgid_entry> >; _Tp = pid_pgid_entry]’:
/usr/include/c++/4.7/bits/stl_algo.h:2315:70:   required from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pid_pgid_entry*, std::vector<pid_pgid_entry> >]’
/usr/include/c++/4.7/bits/stl_algo.h:2347:54:   required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<pid_pgid_entry*, std::vector<pid_pgid_entry> >; _Size = long int]’
/usr/include/c++/4.7/bits/stl_algo.h:5483:4:   required from ‘void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<pid_pgid_entry*, std::vector<pid_pgid_entry> >]’
/home/emaisin/src/binutils-gdb/gdb/nat/linux-osdata.c:480:57:   required from here
/usr/include/c++/4.7/bits/stl_algo.h:2277:4: error: passing ‘const pid_pgid_entry’ as ‘this’ argument of ‘bool pid_pgid_entry::operator<(const pid_pgid_entry&)’ discards qualifiers [-fpermissive]

Making the operator< method const fixes it.

gdb/ChangeLog:

	* nat/linux-osdata.c (struct pid_pgid_entry) <operator<>: Make
	const.