Hide internal __gettextparse function [BZ #18822]

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

	[BZ #18822]
	* include/plural-exp.h: New file.
	* intl/plural-exp.c: Include <plural-exp.h> instead of
	"plural-exp.h".