Silence compiler warning in sepgsql

Silence compiler warning in sepgsql

<selinux/label.h> includes <stdbool.h>, which creates an incompatible
We don't care if <stdbool.h> redefines "true"/"false"; those are close
enough.

Complaint and initial patch by Mike Palmiotto. Final approach per
Tom Lane's suggestion, as discussed on hackers. Backpatching to
all supported branches.

Discussion: https://postgr.es/m/flat/623bcaae-112e-ced0-8c22-a84f75ae0c53%40joeconway.com