jail: Mute -Wunused-but-set-variable for yynerrs.

This is in the generated parser when using %pure-parser.

y.tab.c:382:14: error: variable 'yynerrs' set but not used [-Werror,-Wunused-but-set-variable]

int      yynerrs;
         ^

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40671