Fix build on FreeBSD (again).

Broken by commit a6ae3e7a, which added sysctlbyname() without
required headers.

While there, remove unused PATH_MAX2.

--
MOS_MIGRATED_REVID=119619207
diff --git a/src/main/native/unix_jni_freebsd.cc b/src/main/native/unix_jni_freebsd.cc
index 2ed1b2a..5be1d4d 100644
--- a/src/main/native/unix_jni_freebsd.cc
+++ b/src/main/native/unix_jni_freebsd.cc
@@ -22,11 +22,11 @@
 #include <sys/extattr.h>
 #include <sys/param.h>
 #include <sys/stat.h>
+#include <sys/sysctl.h>
+#include <sys/types.h>
 
 #include <string>
 
-const int PATH_MAX2 = PATH_MAX * 2;
-
 using std::string;
 
 // See unix_jni.h.