graphics/glad: Add explicit Python build dependency (D29353)
This unbreaks the build when using devel/samurai.
It previously got it through ninja which has a run dependency on
Python.
[ 0% 1/3] cd /wrkdirs/usr/ports/graphics/glad/work/glad-0.1.34 && _Python_EXECUTABLE-NOTFOUND -m glad --profile=compatibility --out-path=/wrkdirs/usr/ports/graphics/glad/work/.build --api= --generator=c --extensions= --spec=gl --reproducible
samu: job failed: cd /wrkdirs/usr/ports/graphics/glad/work/glad-0.1.34 && _Python_EXECUTABLE-NOTFOUND -m glad --profile=compatibility --out-path=/wrkdirs/usr/ports/graphics/glad/work/.build --api= --generator=c --extensions= --spec=gl --reproducible
/bin/sh: _Python_EXECUTABLE-NOTFOUND: not found
samu: subcommand failed
PR: 254678