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

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-04_07h37m38s/logs/errors/glad-0.1.34.log

PR:		254678

graphics/glad/Makefile

@@ -10,7 +10,7 @@ COMMENT=	Multi-Language Vulkan/GL/GLES/EGL/GLX/WGL loader-generator
 LICENSE=	MIT
 LICENSE_FILE=	${WRKSRC}/LICENSE
 
-USES=		cmake
+USES=		cmake python:build
 USE_GITHUB=	yes
 GH_ACCOUNT=	Dav1dde
 USE_LDCONFIG=	yes