mesa: add opencl -> clang build dependency

mesa: add opencl -> clang build dependency

With the commits 448f4a84cb22 ("llvm: add recipe for just the LLVM
libraries") and d76dc362c8e1 ("clang: use llvm recipe") the 'llvm'
dependency inside mesa.inc does no longer pull in the clang libraries,
failing RustiCL build as it can not find Clang libaries.

Add direct dependency on the clang in order to fulfill build-time deps.

Fixes: d76dc362c8e1 ("clang: use llvm recipe")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>