security/p5-GSSAPI: Fix build on 16-CURRENT and 15-STABLE with MIT KRB5

When WITH_MITKRB5 is enabled (default) on 15-STABLE and 16-CURRENT
this port fails to build, expecting Heimdal in base. Fix this.

PR: 289290
Rported by: Hyogeol Lee <hyogeollee@gmail.com>
Patch by: Hyogeol Lee <hyogeollee@gmail.com>