src: remove unused #include statement

strcasecmp() is not used in src/node_http_parser.cc so there is no need
to include its header file.

PR-URL: https://github.com/nodejs/node/pull/6582


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>