Remove vestiges of external build support from libio headers.

As requested by Adhemerval, this patch removes some preprocessor
conditionals from the libio headers that were only relevant when
building libio outside glibc.

Installed stripped libraries and executables are unchanged by this
patch.

	* libio/iolibio.h, libio/libioP.h: Remove extern "C".
	* libio/libio.h: Remove __BEGIN_DECLS and __END_DECLS.
	Remove preprocessor conditionals on _LIBC and __USE_GNU,
	which are always true, and __cplusplus, which is always false.