Import SwiftProtobuf in generated code (#952)

Motivation:

Generated code may rely on messages which ship with SwiftProtobuf, as
such we should always import SwiftProtobuf.

Modifications:

- Import SwiftProtobuf in codegen

Result:

Resolves #951

Sources/protoc-gen-grpc-swift/Generator.swift

@@ -90,6 +90,7 @@ class Generator {
     let moduleNames = [
       self.options.gRPCModuleName,
       "NIO",
+      "SwiftProtobuf",
     ]
 
     for moduleName in (moduleNames + self.options.extraModuleImports).sorted() {