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

lib/http_negotiate.c

@@ -36,7 +36,7 @@
 
 #include "urldata.h"
 #include "sendf.h"
-#include "gssapi.h"
+#include "curl_gssapi.h"
 #include "rawstr.h"
 #include "curl_base64.h"
 #include "http_negotiate.h"