[xcodegen] Include Obj-C(++) sources

Can't believe I missed these for so long.

Author: hamishknight

utils/swift-xcodegen/Sources/SwiftXcodeGen/Path/FileExtension.swift

@@ -16,7 +16,9 @@ public enum FileExtension: String {
   case def
   case gyb
   case h
+  case m
   case md
+  case mm
   case modulemap
   case o
   case rst

utils/swift-xcodegen/Sources/SwiftXcodeGen/Path/PathProtocol.swift

@@ -124,7 +124,7 @@ extension PathProtocol {
   }
 
   var isCSourceLike: Bool {
-    hasExtension(.c, .cpp)
+    hasExtension(.c, .cpp, .m, .mm)
   }
 
   var isDocLike: Bool {