[Freeciv-Dev] (PR#12424) load_sprite should be static

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=12424 >

Nobody outside of tileset.c should ever use load_sprite.  All of the 
users I know of now are buggy and should be fixed.

As an example the gtk2 client uses this function to load a flag sprite. 
  But it doesn't check the fallback flag so it may end up with a NULL 
flag, when it could easily just access the already-loaded flag directly.

-jason