Fix i386 MINIMAL build

Include systm.h in i386's counter.h, so it can use critical_*().

In file included from /usr/src/sys/netinet6/ip6_var.h:250:
In file included from /usr/src/sys/sys/counter.h:37:
./machine/counter.h:191:3: error: call to undeclared function 'critical_enter'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]

critical_enter();
^

./machine/counter.h:193:3: error: call to undeclared function 'critical_exit'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]

critical_exit();
^