Import fs::SeekMode enum into global namespace

Author: igrr

hardware/esp8266com/esp8266/cores/esp8266/FS.h

@@ -105,14 +105,17 @@ class FS
 
 protected:
     FSImplPtr _impl;
-
 };
 
 } // namespace fs
 
 using fs::FS;
 using fs::File;
 using fs::Dir;
+using fs::SeekMode;
+using fs::SeekSet;
+using fs::SeekCur;
+using fs::SeekEnd;
 
 extern FS SPIFFS;