Default mesa driver for ubuntu 20.04 fails when a name is defined twice.
M_PI is defined in both common_workbench_lib and common_math_lib. This patch
remove the define out of common_workbench_lib

For reference it fails on https://github.com/mesa3d/mesa/blob/mesa-20.0.8/src/compiler/glsl/glcpp/glcpp-parse.y#L1186
during the check if the macros are the same.