sasl: Moved Curl_sasl_gssapi_cleanup() definition into header file

Rather than define the function as extern in the source files that use
it, moved the function declaration into the SASL header file just like
the Digest and NTLM clean-up functions.

Additionally, added a function description comment block.

lib/curl_sasl_sspi.c

@@ -47,10 +47,6 @@
 /* The last #include file should be: */
 #include "memdebug.h"
 
-#if defined(USE_KRB5)
-void Curl_sasl_gssapi_cleanup(struct kerberos5data *krb5);
-#endif
-
 /*
  * Curl_sasl_build_spn()
  *
@@ -1010,6 +1006,16 @@ CURLcode Curl_sasl_create_gssapi_security_message(struct SessionHandle *data,
   return result;
 }
 
+/*
+* Curl_sasl_gssapi_cleanup()
+*
+* This is used to clean up the gssapi specific data.
+*
+* Parameters:
+*
+* krb5     [in/out] - The kerberos 5 data struct being cleaned up.
+*
+*/
 void Curl_sasl_gssapi_cleanup(struct kerberos5data *krb5)
 {
   /* Free our security context */