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

usr.sbin/jail/Makefile

@@ -11,6 +11,7 @@ LIBADD=	jail kvm util
 PACKAGE=jail
 
 NO_WMISSING_VARIABLE_DECLARATIONS=
+CWARNFLAGS.jailparse.c=	${NO_WUNUSED_BUT_SET_VARIABLE}
 
 YFLAGS+=-v
 CFLAGS+=-I. -I${.CURDIR}