[ExecutionEngine] Fix a warning

This patch fixes:

  llvm/lib/ExecutionEngine/Orc/TargetProcess/UnwindInfoManager.cpp:53:20:
  error: unused variable 'AddFnName' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/ExecutionEngine/Orc/TargetProcess/UnwindInfoManager.cpp

@@ -50,7 +50,7 @@ llvm_orc_rt_alt_UnwindInfoManager_deregister(const char *Data, uint64_t Size) {
 
 namespace llvm::orc {
 
-static const char *AddFnName = "__unw_add_find_dynamic_unwind_sections";
+[[maybe_unused]] static const char *AddFnName = "__unw_add_find_dynamic_unwind_sections";
 [[maybe_unused]] static const char *RemoveFnName = "__unw_remove_find_dynamic_unwind_sections";
 static std::unique_ptr<UnwindInfoManager> Instance;
 static int (*RemoveFindDynamicUnwindSections)(void *) = nullptr;