libvlr: fix sgsn tmsi creation, replace constant with define

reported by _dev_zero in #osmocom

Change-Id: Ib5679ab5d06b6ef735725b4a68eeb1e9cbcc11ba
Depends-On: libosmocore I52b9f6b5f3e96d85a390ba2af21d7814df8aaeec
1 file changed