gdbsupport: use result_of_t instead of result_of in parallel-for.h

gdbsupport: use result_of_t instead of result_of in parallel-for.h

When building with -std=c++11, I get:

In file included from /home/smarchi/src/binutils-gdb/gdb/unittests/parallel-for-selftests.c:22:                                                                             /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/parallel-for.h:134:10: error: ‘result_of_t’ is not a member of ‘std’; did you mean ‘result_of’?
  134 |     std::result_of_t<RangeFunction (RandomIt, RandomIt)>
      |          ^~~~~~~~~~~
      |          result_of

This is because result_of_t has been introduced in C++14.  Use the
equivalent result_of<...>::type instead.

result_of and result_of_t have been removed in C++20 though, so I think
we'll need some patches eventually to make the code use invoke_result
instead, depending on the C++ version.

Change-Id: I4817f361c0ebcdd4b32976898fc368bb302b61b9