http: Don't expose function definitions when components are disabled #3861
As per other header files. For example: http.h and http_ntlm.h.
There is no point in exposing the functions definitions unnecessarily when components are disabled.
This is more of a tidy up for consistency rather than to fix an actual (existing) issue. As the current code stands these header files would only cause a problem if a libcurl developer tried to a) include them in non HTTP code or b) use the HTTP negotiate functions from outside the negotiate code in http.c - prior to this commit this would cause a linker error rather than a compilation error as the function would be defined but not exist.