Fix the MS-Windows build due to added GnuTLS functions

Fix the MS-Windows build due to added GnuTLS functions

* src/gnutls.c [WINDOWSNT]: Add DEF_DLL_FN for new functions.
(init_gnutls_functions) [WINDOWSNT]: Add LOAD_DLL_FN for new
functions.  Add #define redirections for new functions.
(gnutls_symmetric_aead): Fix format specs to be more portable when
printing ptrdiff_t arguments.
* src/fns.c (gnutls_rnd) [WINDOWSNT]: Redirect to w32_gnutls_rnd
wrapper.
* src/gnutls.h [WINDOWSNT]: Add prototype for w32_gnutls_rnd.

* test/lisp/net/gnutls-tests.el (gnutls-tests-tested-macs)
(gnutls-tests-tested-digests, gnutls-tests-tested-ciphers): Call
gnutls-available-p, otherwise GnuTLS functions might not be loaded
from the DLL on MS-Windows.