NFC, Avoid a warning in WasmObjectWriter

The warning was (introduced in r331220):

lib/MC/WasmObjectWriter.cpp:51:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

llvm-svn: 331251

llvm/lib/MC/WasmObjectWriter.cpp

@@ -47,6 +47,8 @@ static std::string toString(wasm::WasmSymbolType type) {
     return "WASM_SYMBOL_TYPE_DATA";
   case wasm::WASM_SYMBOL_TYPE_SECTION:
     return "WASM_SYMBOL_TYPE_SECTION";
+  default:
+    llvm_unreachable("unexpected kind");
   }
 }