tests: llvm-test-suite: Add missing ninja-build requirement
    
    Note from kkleine:
    
    This is a backport of
    https://src.fedoraproject.org/rpms/clang/c/4cce509493d67e30bdbc6af090feeb27302f5456?branch=rawhide
    to fix the failing /clang-tests/tests/llvm-test-suite test:
    
    ```
    + git clone --depth 1 https://src.fedoraproject.org/rpms/llvm-test-suite.git llvm-test-suite
    Cloning into 'llvm-test-suite'...
    + cd llvm-test-suite/tests/test-suite
    + ./runtest.sh
    ++ mktemp -d
    + cd /tmp/tmp.j8wd1oLfpN
    + cmake -G Ninja /usr/share/llvm-test-suite/ -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DTEST_SUITE_LIT_FLAGS=-sv
    CMake Error: CMake was unable to find a build program corresponding to "Ninja".  CMAKE_MAKE_PROGRAM is not set.  You probably need to select a different build tool.
    -- Configuring incomplete, errors occurred!
    See also "/tmp/tmp.j8wd1oLfpN/CMakeFiles/CMakeOutput.log".
    ```