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)
```
7 jobs for !16315 with darktable in 4 minutes and 10 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
passed #285158
docker-alpine x86_64
lint

00:00:24

 
  Build
passed #285163
docker-alpine ci-build aarch64
build-aarch64

00:02:55

passed #285164
docker-alpine ci-build armv7
build-armv7

00:00:46

passed #285162
docker-alpine ci-build ppc64le
build-ppc64le

00:00:55

passed #285161
docker-alpine ci-build s390x
build-s390x

00:00:40

passed #285160
docker-alpine ci-build x86
build-x86

00:00:22

passed #285159
docker-alpine ci-build x86_64
build-x86_64

00:02:01