filelock_test: add freebsd headers for waitpid (#7010)

Summary:
Per manual https://www.unix.com/man-page/FreeBSD/2/waitpid
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7010

Reviewed By: siying

Differential Revision: D22176164

Pulled By: ajkr

fbshipit-source-id: 0a850ae6f1791d10951d5e4a79cfee01a3981d5a

util/filelock_test.cc

@@ -7,6 +7,10 @@
 #include "rocksdb/env.h"
 
 #include <fcntl.h>
+#ifdef __FreeBSD__
+#include <sys/types.h>
+#include <sys/wait.h>
+#endif
 #include <vector>
 #include "test_util/testharness.h"
 #include "util/coding.h"