Ensure proc-service symbols have default visibility (PR build/24805)

Compiling GDB with '-fvisibility=hidden' removes the symbols that
should be exported.

This patch explicitly marks them as visible.

gdb/ChangeLog:
2020-01-16  Pedro Alves  <palves@redhat.com>
            Norbert Lange  <nolange79@gmail.com>

	PR build/24805
	* gdbsupport/gdb_proc_service.h (PS_EXPORT): New.
	(ps_get_thread_area, ps_getpid, ps_lcontinue, ps_lgetfpregs)
	(ps_lgetregs, ps_lsetfpregs, ps_lsetregs, ps_lstop, ps_pcontinue)
	(ps_pdread, ps_pdwrite, ps_pglobal_lookup, ps_pstop, ps_ptread)
	(ps_ptwrite, ps_lgetxregs, ps_lgetxregsize, ps_lsetxregs)
	(ps_plog): Redeclare exported functions with default visibility.