Bug#1029057: llvm-toolchain-15 cannot be built without git

[Jian-Hon Pan]

Package: llvm-toolchain-15
Version: 1:15.0.6-4

When I try to build llvm-toolchain-15 by myself, it shows the error:

CMake Warning at cmake/modules/VersionFromVCS.cmake:49 (message):
  Git not found.  Version cannot be determined.
Call Stack (most recent call first):
  CMakeLists.txt:992 (get_source_info)
  
Then, I trace the code [1] and make sure it needs git for build.
I think git should be in the Build-Depends of llvm-toolchain-15, too.

[1]: https://github.com/llvm/llvm-project/blob/8dfdcc7b7bf66834a761bd8de445840ef68e4d1a/llvm/cmake/modules/VersionFromVCS.cmake#L7-L50

Jian-Hong Pan