ibstat: Include prototype for sysctlbyname().

Fixes the following compile warning:
      implicit declaration of function 'sysctlbyname' is invalid in C99
      [-Wimplicit-function-declaration]

Found by:	J87
Differential Revision:	https://reviews.freebsd.org/D30484
MFC after:	1 week
Sponsored by:	Mellanox Technologies // NVIDIA Networking

contrib/ofed/infiniband-diags/src/ibstat.c

@@ -48,6 +48,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <sys/sysctl.h>
 
 #include <infiniband/umad.h>