Fix that placeholderTextColor doesn't exist in iOS SDK < 13

Author: devxoul

Sources/UITextView+Placeholder.m

@@ -55,7 +55,10 @@ - (void)swizzledDealloc {
 
 + (UIColor *)defaultPlaceholderColor {
     if (@available(iOS 13, *)) {
-      return [UIColor placeholderTextColor];
+      SEL selector = NSSelectorFromString(@"placeholderTextColor");
+      if ([UIColor respondsToSelector:selector]) {
+        return [UIColor performSelector:selector];
+      }
     }
     static UIColor *color = nil;
     static dispatch_once_t onceToken;