fix lCoin compilation issue (obsoleted by !26)

Hi @rbianchi, @dellacqu,

I tried to setup GeoModel from scratch today on Mac Mojave 10.14.6 (where the setup was working fine previously), but during compilation:

cmake -DCMAKE_INSTALL_PREFIX=../install/ -DGEOMODEL_BUILD_VISUALIZATION=1 -DGEOMODEL_USE_BUILTIN_JSON=TRUE -DGEOMODEL_BUILD_FULLSIMLIGHT=1 ../GeoModel/
make

i.e. during the make step, I encountered:

ld: library not found for -lCoin
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [GeoModelVisualization/VP1Base/libGXBase.1.0.0.dylib] Error 1
make[1]: *** [GeoModelVisualization/VP1Base/CMakeFiles/GXBase.dir/all] Error 2

I noticed, that when I revert @rbianchi's change of GeoModelVisualization/VP1Base/CMakeLists.txt only, the compilation error goes away. However, I did not yet understand why this is the case and seems to work for you on newer MacOS versions?

Best, Nico