gprs_sgsn.h: fix wrong type of re-attempts

The GMM ctx->gmm_att_req.auth_reattempt is used to track
multiple UTMS re-sync attempt of a MS.

Change-Id: I708226cec9e131dcda4234f42ed3689f4f6750e8
Fixes: f7198d7dbb84 ("gprs_gmm: introduce a GMM Attach Request FSM")
Fixes: OS#3556