Fix i386 build after put*ent hardening changes.

The recent put*ent hardening changes broke the build for i386.  i386
defines internal_function to __attribute__ ((regparm (3), stdcall)),
which affects type compatibility, so requiring internal_function to be
used consistently on declarations and definitions.  This patch adds
internal_function to the definitions of the new functions using it on
their declarations.

Tested for i386 that this fixes the build.

	* nss/rewrite_field.c (__nss_rewrite_field): Use
	internal_function.
	* nss/valid_field.c (__nss_valid_field): Likewise.
	* nss/valid_list_field.c (__nss_valid_list_field): Likewise.