pgsql: Fix compilation warnings with libselinux 3.1 in contrib/sepgsql/

Fix compilation warnings with libselinux 3.1 in contrib/sepgsql/

Upstream SELinux has recently marked security_context_t as officially
deprecated, causing warnings with -Wdeprecated-declarations. This is
considered as legacy code for some time now by upstream as
security_context_t got removed from most of the code tree during the
development of 2.3 back in 2014.

This removes all the references to security_context_t in sepgsql/ to be
consistent with SELinux, fixing the warnings. Note that this does not
impact the minimum version of libselinux supported.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20200813012735.GC11663@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1f32136a9960df2e135e7b36363ea1f087b514a0

Modified Files
--------------
contrib/sepgsql/label.c | 10 +++++-----
contrib/sepgsql/selinux.c | 10 +++++-----
contrib/sepgsql/uavc.c | 4 ++--
3 files changed, 12 insertions(+), 12 deletions(-)