Update lit.site.cfg.py.in

Author: makslevental

mlir/test/lit.site.cfg.py.in

@@ -4,6 +4,7 @@ import sys
 
 config.target_triple = "@LLVM_TARGET_TRIPLE@"
 config.llvm_src_root = "@LLVM_SOURCE_DIR@"
+config.llvm_obj_root = "@LLVM_BINARY_DIR@"
 config.llvm_tools_dir = lit_config.substitute("@LLVM_TOOLS_DIR@")
 config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
 config.spirv_tools_tests = @LLVM_INCLUDE_SPIRV_TOOLS_TESTS@