parser: add a workaround for byacc

Unlike bison, byacc outputs its own parser code *after* our own parser.y
code, which includes the #undef. So this fix is needed for the 'scanner'
-> 'param->scanner' translation to work in the parser.c code generated
by byacc.

Signed-off-by: Ran Benita <ran234@gmail.com>

src/xkbcomp/parser.y

@@ -798,3 +798,5 @@ parse(struct xkb_context *ctx, void *scanner, const char *map)
 
     return first;
 }
+
+#define scanner param->scanner