[WebAssembly] Remove `using` statements from header files. NFC.

Differential Revision: https://reviews.llvm.org/D54758

llvm-svn: 347621
diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
index a42e363..9c967ff 100644
--- a/lld/wasm/SymbolTable.cpp
+++ b/lld/wasm/SymbolTable.cpp
@@ -20,6 +20,7 @@
 
 using namespace llvm;
 using namespace llvm::wasm;
+using namespace llvm::object;
 using namespace lld;
 using namespace lld::wasm;