gssapi: rename our files to avoid conflicts

gssapi.h is used as a header name by Heimdal-style GSSAPI so it would
conflict with a private header using that name, and while renaming the
header I figured we should name the .c file accordingly as well.

Bug: http://curl.haxx.se/mail/lib-2011-07/0071.html
Reported by: Ben Greear