Hide internal __get_sol function [BZ #18822]

Hide internal __get_sol function to allow direct access within libc.so
and libc.a without using GOT nor PLT.

	[BZ #18822]
	* sysdeps/unix/sysv/linux/getsourcefilter.c: Include
	"getsourcefilter.h".
	* sysdeps/unix/sysv/linux/getsourcefilter.h: New file.
	* sysdeps/unix/sysv/linux/setsourcefilter.c: Include
	"getsourcefilter.h".
	(__get_sol): Removed.