Fix iOS build target

RSXML/RSOPMLItem.m

@@ -135,6 +135,8 @@ NSString *OPMLXMLURLKey = @"xmlUrl";
 	return mStr;
 }
 
+#ifdef TARGET_MAC
+
 /// Can be used to export directly to @c .opml file.
 - (NSXMLDocument *)exportXML {
 	NSXMLElement *head = [NSXMLElement elementWithName:@"head"];
@@ -172,4 +174,6 @@ NSString *OPMLXMLURLKey = @"xmlUrl";
 	}
 }
 
+#endif
+
 @end