update test imports

Author: camden-king

GoogleSignIn/Tests/Unit/GIDAuthStateMigrationTest.m

@@ -16,7 +16,9 @@
 
 #import "GoogleSignIn/Sources/GIDAuthStateMigration.h"
 #import "GoogleSignIn/Sources/GIDSignInCallbackSchemes.h"
+#if TARGET_OS_OSX || TARGET_OS_MACCATALYST
 #import "GoogleSignIn/Tests/Unit/OIDAuthState+Testing.h"
+#endif
 
 @import GTMAppAuth;