Fix disable_signup paramter conditional

Author: Julian Locke

Source/ObjectiveDropboxOfficial/Shared/Handwritten/OAuth/DBOAuthManager.m

@@ -137,7 +137,7 @@ - (instancetype)initWithAppKey:(NSString *)appKey host:(NSString *)host redirect
     _cancelURL = [NSURL URLWithString:[NSString stringWithFormat:@"db-%@://2/cancel", _appKey]];
     _host = host;
     _urls = [NSMutableArray arrayWithObjects:_redirectURL, nil];
-#ifdef TARGET_OS_OSX
+#if TARGET_OS_OSX
     _disableSignup = NO;
 #else
     _disableSignup = YES;