Use NSURLIsExcludedFromBackupKey without crashing on iOS 5.0

#include <sys/xattr.h>

-(BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL
{

constchar* filePath =[[URL path] fileSystemRepresentation];
constchar* attrName ="com.apple.MobileBackup";
if(&NSURLIsExcludedFromBackupKey==nil){
    // iOS 5.0.1 and lower
    u_int8_t attrValue =1;
    int result = setxattr(filePath, attrName,&attrValue,sizeof(attrValue),0,0);
    return result ==0;

}
else{
    // First try and remove the extended attribute if it is present
    int result = getxattr(filePath, attrName, NULL,sizeof(u_int8_t),0,0);
    if(result !=-1){
        // The attribute exists, we need to remove it
        int removeResult = removexattr(filePath, attrName,0);
        if(removeResult ==0){
            NSLog(@"Removed extended attribute on file %@", URL);
        }
    }

    // Set the new key
    NSError*error =nil;
    [URL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:&error];
    return error ==nil;
}
}