[WebAssembly] Fix import type to be signed LEBs

[sbc100]

Summary

This fix is a follow up a previous change with stored
value types as signed integers in memory.

In future, once the yaml<->wasm binary patche lands we
can add test coverage for this kind of thing.

lib/MC/WasmObjectWriter.cpp

struct WasmFunctionTypeDenseMapInfo {
   }
 };
 
 // A wasm import to be written into the import section.
 struct WasmImport {
   StringRef ModuleName;
   StringRef FieldName;
   unsigned Kind;
-  uint32_t Type;
+  int32_t Type;
 };
 
 // A wasm function to be written into the function section.
 struct WasmFunction {
   int32_t Type;
   const MCSymbolWasm *Sym;
 };