graphics/mesa-devel: unbreak standalone after e502b6db

llvm-config found: NO found '9.0.1' but need '>= 11.0.0'
Run-time dependency LLVM found: NO (tried cmake and config-tool)
Looking for a fallback subproject for the dependency llvm (modules: bitwriter, engine, mcdisassembler, mcjit, core, executionengine, scalaropts, transformutils, instcombine, amdgpu, native, bitreader, ipo, asmparser)

meson.build:1604:2: ERROR: Neither a subproject directory nor a llvm.wrap file was found.