Fix a compilation error in screen.c

On OS X Mavericks the compilations fails with the following error:

gcc  -I/opt/local/include -I/opt/local/include/freetype2 -Wall -fPIC -I/usr/X11/include   -L/opt/local/lib  -o mrxvt  command.o init.o menubar.o strings.o encoding.o misc.o ptytty.o tabbar.o netdisp.o rxvt.o xdefaults.o main.o pixmap.o screen.o scrollbar.o scrollbar-plain.o scrollbar-xterm.o scrollbar-next.o scrollbar-rxvt.o scrollbar-sgi.o logging.o   mjpg.o  session.o  -lXft -lfontconfig -ljpeg -lXpm -lICE -lSM -lX11
Undefined symbols for architecture x86_64:
  "_rxvt_clear_area", referenced from:
      _rxvt_scr_refresh in screen.o
      _rxvt_scr_erase_screen in screen.o
  "_rxvt_fill_rectangle", referenced from:
      _rxvt_scr_refresh in screen.o
      _rxvt_scr_erase_screen in screen.o
      _rxvt_scr_draw_string in screen.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [mrxvt] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2

A way to fix this is to declare as static the two functions. This was first
suggested by Quintin Beukes for mrxvt 0.4.2 in the following email thread:

  http://sourceforge.net/p/materm/mailman/message/3390648/