HTTP: include SSI header only where it's needed.

Previously, ngx_http_ssi_filter_module.h (optional HTTP module)
was pulled in by ngx_http.h (part of HTTP core).

Change-Id: Ie6fcc40037866b7ceb9755e0260b344d410b9235
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
Reviewed-on: https://nginx-review.googlesource.com/1007
Reviewed-by: Martin Maly <mmaly@google.com>