Move gdb_select.h to gdbsupport/

Move gdb_select.h to gdbsupport/

This moves gdb_select.h to gdbsupport/, so it can be used by other
code there.

gdb/ChangeLog
2020-04-13  Tom Tromey  <tom@tromey.com>

	* gdb_select.h: Move to ../gdbsupport/.
	* event-loop.c: Update include path.
	* top.c: Update include path.
	* ser-base.c: Update include path.
	* ui-file.c: Update include path.
	* ser-tcp.c: Update include path.
	* guile/scm-ports.c: Update include path.
	* posix-hdep.c: Update include path.
	* ser-unix.c: Update include path.
	* gdb_usleep.c: Update include path.
	* mingw-hdep.c: Update include path.
	* inflow.c: Update include path.
	* infrun.c: Update include path.
	* event-top.c: Update include path.

gdbsupport/ChangeLog
2020-04-13  Tom Tromey  <tom@tromey.com>

	* gdb_select.h: Move from ../gdb/.