[ffigen] NativeFunc's child should be a FunctionType (#461)

pkgs/ffigen/lib/src/code_generator/func_type.dart

diff --git a/pkgs/ffigen/lib/src/code_generator/func_type.dart b/pkgs/ffigen/lib/src/code_generator/func_type.dart
index 0b8abfd..3025bd2 100644
--- a/pkgs/ffigen/lib/src/code_generator/func_type.dart
+++ b/pkgs/ffigen/lib/src/code_generator/func_type.dart
@@ -58,7 +58,7 @@
 
 /// Represents a NativeFunction<Function>.
 class NativeFunc extends Type {
-  final Type type;
+  final FunctionType type;
 
   NativeFunc(this.type);