xapp_sessreg: fix build issue with gcc 5

Backport an upstream commit that fixes the build of xapp_sessreg with
gcc 5.

Fixes:

  http://autobuild.buildroot.org/results/ec0/ec01564854f0738df05b8d995d9f0db396a0e5a8/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>