fix typo in mesa CMakeLists

@ben.boeckel there was a typo in !595 (merged)

+2

Thanks!

Do: merge

mentioned in commit d95bc085

merged

mentioned in commit ben.boeckel/paraview-superbuild@75402d31

mentioned in commit ben.boeckel/paraview-superbuild@46b99089

mentioned in commit ben.boeckel/paraview-superbuild@65c27dd0

mentioned in commit ben.boeckel/paraview-superbuild@e3231d5b

mentioned in commit bob.obara/cmb-superbuild@2ab05153

mentioned in commit bob.obara/cmb-superbuild@37431a73