add note to packagers.md about libseccomp version

Signed-off-by: Jessica Frazelle <acidburn@docker.com>

project/PACKAGERS.md

@@ -58,6 +58,7 @@ To build the Docker daemon, you will additionally need:
   2.02.89 or later
 * btrfs-progs version 3.16.1 or later (unless using an older version is
   absolutely necessary, in which case 3.8 is the minimum)
+* libseccomp version 2.2.1 or later (for build tag seccomp)
 * yubico-piv-tool version 1.1.0 or later (for experimental)
 
 Be sure to also check out Docker's Dockerfile for the most up-to-date list of