XQuartz: Fix darwinfb.h header guard
./darwinfb.h:28:9: warning: '_DARWIN_FB_H' is used as a header guard here, followed by #define of a different macro
[-Wheader-guard,Lexical or Preprocessor Issue]
^~~~~~~~~~~~
./darwinfb.h:29:9: note: '_DARWIN_DB_H' is defined here; did you mean '_DARWIN_FB_H'? [Lexical or Preprocessor Issue]
^~~~~~~~~~~~
_DARWIN_FB_H
Signed-off-by:
Jeremy Huddleston Sequoia <jeremyhu@apple.com>