Move include block to pathstuff.h

A recent commit caused pathstuff.cc to fail to compile on mingw, like:

../../binutils-gdb/gdbsupport/pathstuff.cc:324:1: error: no previous declaration for 'std::string find_gdb_home_config_file(const char*, _stati64*)' [-Werror=missing-declarations]

Some newly-added #includes were changing which "stat" was being seen
by the compiler.  This patch moves the includes to the header, so that
the declaration and definition now agree.

2020-11-10  Tom Tromey  <tromey@adacore.com>

	PR build/26848:
	* pathstuff.h: Move include block here...
	* pathstuff.cc: ... from here.