Fix -Werror=declaration-after-statement build failure on Solaris

Fix -Werror=declaration-after-statement build failure on Solaris

dbus-sysdeps-unix.c: In function ‘_dbus_read_credentials_socket’:
dbus-sysdeps-unix.c:2061:9: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
         adt_session_data_t *adth = NULL;
         ^

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=102145
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: Simon McVittie <smcv@collabora.com>