selftests: timestamping: Fix SIOCGSTAMP undeclared build failure

Add missing linux/sockios.h include to fix the following SIOCGSTAMP
undeclared build error. In addition, remove the local defines for
SIOCGSTAMPNS and SIOCSHWTSTAMP and pick them up from linux/sockios.h.

timestamping.c:249:19: error: SIOCGSTAMP undeclared
   if (ioctl(sock, SIOCGSTAMP, &tv))

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>