Add missing include.

Change-Id: I0aab9c94fcfa58b9cd46eaf716d9317f532f79a2
Reviewed-on: https://boringssl-review.googlesource.com/11850
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/pool/pool_test.cc

@@ -12,6 +12,7 @@
  * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
+#include <stdio.h>
 #include <string.h>
 
 #include <openssl/pool.h>