SwiftMix: properly export SwiftMain method

Tests/SwiftMix/CMakeLists.txt

@@ -3,3 +3,4 @@ project(SwiftMix C Swift)
 
 add_executable(SwiftMix CMain.c ObjCMain.m SwiftMain.swift ObjC-Swift.h)
 set_property(TARGET SwiftMix PROPERTY XCODE_ATTRIBUTE_SWIFT_OBJC_BRIDGING_HEADER "ObjC-Swift.h")
+target_compile_options(SwiftMix PRIVATE "$<$<COMPILE_LANGUAGE:C>:-Werror=objc-method-access>")

Tests/SwiftMix/SwiftMain.swift

@@ -1,7 +1,7 @@
 import Foundation
 
 @objc class SwiftMainClass : NSObject {
-  class func SwiftMain() -> Int32 {
+  @objc class func SwiftMain() -> Int32 {
     dump("Hello World!");
     return 0;
   }