fix build of security/metasploit on 9.x

According to [1] the PBI build is broken. The fix from [2] should work here, too.

[1] http://ftp.pcbsd.org/pub/pbioutgoing/amd64/9/security/metasploit/last_100.log
[2] https://github.com/pfsense/pfsense-packages/pull/649