Always define `gnutls-available-p' even if GnuTLS is not available.

Fixes: debbugs:19346

* gnutls.c (Fgnutls_available_p syms_of_gnutls): Move later for
clarity.  Let the availability check return Qnil when the GnuTLS
integration is not available, instead of erroring out.

* gnutls.h: Always declare syms_of_gnutls.

* emacs.c (main): Always include gnutls.h and run syms_of_gnutls.