glsl: Fail the build if the grammar contains shift/reduce errors.

When working on a parser, it's very easy to accidentally introduce
new shift/reduce conflicts.  Failing the build guarantees they'll
be noticed and fixed.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

src/glsl/glsl_parser.yy

@@ -41,6 +41,8 @@ static void yyerror(YYLTYPE *loc, _mesa_glsl_parse_state *st, const char *msg)
 }
 %}
 
+%expect 0
+
 %pure-parser
 %error-verbose