science/xtb: Unbreak build with GCC 10

/wrkdirs/usr/ports/science/xtb/work/xtb-6.3.0/src/prog/main.F90:626:39:

626 |          call relaxed_scan(env,mol,wfn,calc)
    |                                       1

Error: Explicit interface required for polymorphic argument at (1)

http://package18.nyi.freebsd.org/data/121amd64-default-PR244494/2020-06-10_18h04m44s/logs/errors/xtb-6.3.0.log

PR: 246700