ntlm: move NTLM_NEEDS_NSS_INIT define into core NTLM header

.. and include the core NTLM header in all NTLM-related source files.

Follow up to 6f86022d. Since then http_ntlm checks NTLM_NEEDS_NSS_INIT
but did not include vtls.h where it was defined.

Closes https://github.com/curl/curl/pull/1911