build: fix build of virt-login-shell on systems with older gnutls

On systems where gnutls uses libgcrypt, I'm seeing the following
build failure

libvirt.c:314: error: variable 'virTLSThreadImpl' has initializer but incomplete type
libvirt.c:319: error: 'GCRY_THREAD_OPTION_PTHREAD' undeclared here (not in a function)
...

Fix by undefining WITH_GNUTLS_GCRYPT in config-post.h

config-post.h

@@ -34,6 +34,7 @@
 # undef WITH_CURL
 # undef WITH_DTRACE_PROBES
 # undef WITH_GNUTLS
+# undef WITH_GNUTLS_GCRYPT
 # undef WITH_MACVTAP
 # undef WITH_NUMACTL
 # undef WITH_SASL