community/darktable: fix build by disabling switch that requires LLVM
Otherwise we get this error:
Call Stack (most recent call first):
CMakeLists.txt:311 (find_llvm)
CMake Error at /usr/lib/llvm10/lib/cmake/llvm/LLVMExports.cmake:1276 (message):
The imported target "LLVMDemangle" references the file
"/usr/lib/llvm10/lib/libLLVMDemangle.a"
but this file does not exist. Possible reasons include:
* The file was deleted, renamed, or moved to another location.
* An install or uninstall procedure did not complete successfully.
* The installation package was faulty and contained
"/usr/lib/llvm10/lib/cmake/llvm/LLVMExports.cmake"
but not all the files it references.
Call Stack (most recent call first):
/usr/lib/cmake/llvm/LLVMConfig.cmake:251 (include)
CMakeLists.txt:303 (find_package)