meson: Fix DragonFly BSD identifier in meson build files

This changes the operating system identifier tested against
host_machine.system() in meson build files from "dragonflybsd"
to the officially stable "dragonfly".

Signed-off-by: George Matsumura <gmmatsumura01@bvsd.org>
4 jobs for fix_dragonfly_identifier in 6 minutes and 2 seconds
Status Job ID Name Coverage
  Docker Image
passed debian-buster #8646864

00:00:11

 
  Build And Test
passed autotools #8646865

00:05:51

passed meson #8646866

00:02:03

passed meson-noglamor #8646867

00:02:19