Ensure LLVM_ENABLE_ZLIB is always enabled

Set LLVM_ENABLE_ZLIB=FORCE_ON in order to cause the build to error if
zlib is not found.

Author: tuliom

llvm.spec

@@ -974,7 +974,7 @@ popd
 	-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON \\\
 	-DLLVM_ENABLE_PROJECTS="%{projects}" \\\
 	-DLLVM_ENABLE_RUNTIMES="compiler-rt;openmp;offload" \\\
-	-DLLVM_ENABLE_ZLIB:BOOL=ON \\\
+	-DLLVM_ENABLE_ZLIB:BOOL=FORCE_ON \\\
 	-DLLVM_ENABLE_ZSTD:BOOL=FORCE_ON \\\
 	-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=%{experimental_targets_to_build} \\\
 	-DLLVM_INCLUDE_BENCHMARKS=OFF \\\