meson: Fix firewalld check

firewalld is Linux-only, so it should be disabled by default
everywhere else and attempts to explicitly enable firewalld
support on non-Linux targets should result in an error.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

meson.build

@@ -1342,8 +1342,11 @@ endif
 if not get_option('firewalld').disabled()
   firewalld_enable = true
 
-  if host_machine.system() == 'freebsd'
+  if host_machine.system() != 'linux'
     firewalld_enable = false
+    if get_option('firewalld').enabled()
+      error('firewalld support can only be enabled on Linux')
+    endif
   endif
 
   if firewalld_enable