Xext/xf86bigfont.c contains three non-static functions which are called
elsewhere in the server.  This creates a new header containing these
declarations in order to fix several warnings:

xf86bigfont.c:285: warning: no previous prototype for `XF86BigfontFreeFontShm'
dixfonts.c:502: warning: implicit declaration of function `XF86BigfontFreeFontS$
dixfonts.c:502: warning: nested extern declaration of `XF86BigfontFreeFontShm'
log.c:436: warning: implicit declaration of function `XF86BigfontCleanup'
log.c:436: warning: nested extern declaration of `XF86BigfontCleanup'

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Reviewed-by: Julien Cristau <jcristau@debian.org>