Fixed ordering of cmake commands

This (apparently) fixes a current issue with the detection of OpenMesh / ConvexPolyhedron shape in mesa_pd:

• these shapes require OpenMesh but use a separate define WALBERLA_MESAPD_CONVEX_POLYHEDRON_AVAILABLE
• this is set here: https://i10git.cs.fau.de/walberla/walberla/-/blob/master/src/mesa_pd/CMakeLists.txt#L10
• this happened AFTER src/walberlaDefinitions.h -> The corresponding line in this file is not active (i.e., #cmakedefine WALBERLA_MESAPD_CONVEX_POLYHEDRON_AVAILABLE is still commented)

As a result, executing cmake and then make in a fresh build directory leads to the cmake output that the convex shape is available but, when building, it is not found. This was not noticed in the CI since there we call cmake twice, which also "fixes" this issue.

Hopefully this reordering of the cake commands does not break something else.