Hide internal __tcgetattr function [BZ #18822]

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

	[BZ #18822]
	* include/termios.h (__tcgetattr): Add libc_hidden_proto.
	* sysdeps/unix/bsd/tcgetattr.c (__tcgetattr): Add
	libc_hidden_def.
	* sysdeps/unix/sysv/linux/tcgetattr.c (__tcgetattr): Likewise.
	* termios/tcgetattr.c (__tcgetattr): Likewise.