also, i don't see how the problem could stay in the sketch file.
Then, you don't understand the build process.
All files in the sketch directory are copied to a temp, directory for the build process. All include files referenced by the sketch are copied to that temp. directory, too, along with the associated source files.
Only the stuff in that temp. directory is involved in the build.
So, if sketch.pde (or .ino) does not include LiquidCrystal.h, LiquidCrystal.h and LiquidCrystal.cpp are NOT copied to the temp. directory, and are not available for MenuLCD to include.