md5: portability fix -- include stdbool.h explicitly

stdbool.h needs to be included to use type bool variables. Due to
namespace pollution, this gets brought in on FreeBSD, but not on
other systems. Include it explicilty.

Noticed by: arichards@
Sponsored by: Netflix

(cherry picked from commit d0ea5e467f3c44909667c5ee90c3d26653fb6687)