RDMA/uapi: Include socket.h in rdma_user_cm.h

added struct sockaddr_storage to rdma_user_cm.h without also adding an
include for linux/socket.h to make sure it is defined.  Systemtap
needs the header files to build standalone and cannot rely on other
files to pre-include other headers, so add linux/socket.h to the list
of includes in this file.

Fixes: ee7aed45

 ("RDMA/ucma: Support querying for AF_IB addresses")
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>