Improve guard to support concatenated files

ftplugin/zig.vim

@@ -1,7 +1,4 @@
-if exists('g:polyglot_disabled') && index(g:polyglot_disabled, 'zig') != -1
-  finish
-endif
-
+if !exists('g:polyglot_disabled') || index(g:polyglot_disabled, 'zig') == -1
 " Only do this when not done yet for this buffer
 if (exists("b:did_ftplugin"))
   finish
@@ -16,3 +13,4 @@ set shiftwidth=4
 setlocal suffixesadd=.zig
 setlocal commentstring=//\ %s
 setlocal makeprg=zig\ build
+endif