graphics/mesa-dri: unbreak on powerpc* + clang after r457430

python2.7 ./radv_entrypoints_gen.py \

--xml ../../../src/vulkan/registry/vk.xml --outdir .

Traceback (most recent call last):

File "./radv_entrypoints_gen.py", line 30, in <module>
  from mako.template import Template

ImportError: No module named mako.template

PR: 221540
Reported by: Mark Millard (via ports@ list)
Approved by: portmgr blanket