BUILD: haproxy: mark deinit_and_exit() as noreturn

BUILD: haproxy: mark deinit_and_exit() as noreturn

Commit 0a3b43d9c ("MINOR: haproxy: Make use of deinit_and_exit() for
clean exits") introduced this build warning:

  src/haproxy.c: In function 'main':
  src/haproxy.c:3775:1: warning: control reaches end of non-void function [-Wreturn-type]
   }
   ^

This is because the new deinit_and_exit() is not marked as "noreturn"
so depending on the optimizations, the noreturn attribute of exit() will
either leak through it and silence the warning or not and confuse the
compiler. Let's just add the attribute to fix this.

No backport is needed, this is purely 2.2.