tests: Include wait-related headers for Ltest-mem-validate (#74)

Without these, I was getting errors from this test set claiming that
wait, WIFCONTINUED, et al. were undefined on FreeBSD 11.1.

tests/Ltest-mem-validate.c

@@ -35,6 +35,8 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.  */
 
 #include <sys/resource.h>
 #include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 
 #define panic(args...)				\
 	{ fprintf (stderr, args); exit (-1); }