tests: llvm-test-suite: Add missing ninja-build requirement

Note from kkleine:

This is a backport of
4cce509493?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".
```

tests/llvm-test-suite/main.fmf

@@ -13,5 +13,6 @@ test: "$WITH_SCL ./test.sh"
 require:
   - git
   - clang
+  - ninja-build
   - llvm-test-suite
 duration: 1h