Make XkbFileCreate argument types match between header & implementation

Fixes build failure with Solaris Studio compilers:
"src/xkbcomp/ast-build.c", line 492: identifier redeclared: XkbFileCreate
  current : function(..., enum xkb_map_flags)
  previous: function(..., unsigned int) : "src/xkbcomp/ast-build.h", line 98

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>