[mlir] Link MLIRMlirOptMain against test_libs

In 10ef20f I dropped $test_libs
from $LIBS to handle them separately for the mlir-opt tool.
However, they should still include them in LINK_LIBS for the
MLIRMlirOptMain library.

mlir/tools/mlir-opt/CMakeLists.txt

@@ -94,6 +94,7 @@ add_mlir_library(MLIRMlirOptMain
 
   LINK_LIBS PUBLIC
   ${LIBS}
+  ${test_libs}
   )
 
 add_mlir_tool(mlir-opt