Add SEEK_HOLE and SEEK_DATA symbols for older distros.

This patch fixes these build errors on CentOS 6.

  CC	syscalls/lseek.o
syscalls/lseek.c:18: error: ‘SEEK_DATA’ undeclared here (not in a function)
syscalls/lseek.c:18: error: ‘SEEK_HOLE’ undeclared here (not in a function)

Signed-off-by: Vinson Lee <vlee@twitter.com>