Add missing include for waitpid

waitpid is defined in <sys/wait.h>. Reportedly FreeBSD cares about this.

Fixed: 664
Change-Id: I05be6aa50f97dec3ef7098480b8207cffc73e7a2
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63705
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>

crypto/fipsmodule/rand/fork_detect_test.cc

@@ -23,6 +23,7 @@
 #include <inttypes.h>
 #include <stdio.h>
 #include <string.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #include <functional>