[flang] [cmake] Call mlir_target_link_libraries() only with MLIR_LIBS

Do not call `mlir_target_link_libraries()` when there are no `MLIR_LIBS`
specified.  This fixes a failure when `flang/runtime` is being built
standalone, as it does not use MLIR and therefore the function
is not defined:

https://lab.llvm.org/buildbot/#/builders/152/builds/932

Fixes 6a2cc122296b04a4f09768a714a74ffc82b7be87

flang/cmake/modules/AddFlang.cmake

@@ -66,7 +66,9 @@ function(add_flang_library name)
   llvm_add_library(${name} ${LIBTYPE} ${ARG_UNPARSED_ARGUMENTS} ${srcs})
 
   clang_target_link_libraries(${name} PRIVATE ${ARG_CLANG_LIBS})
+  if (ARG_MLIR_LIBS)
     mlir_target_link_libraries(${name} PRIVATE ${ARG_MLIR_LIBS})
+  endif()
 
   if (TARGET ${name})