Don't use Linux-only types in otherwise portable code.

../src/system-prompter.c:25:8: error: unknown type name 'ulong'
static ulong owner_id;
       ^

--- src/system-prompter.c.orig	2020-08-03 08:10:28 UTC
+++ src/system-prompter.c
@@ -22,7 +22,7 @@
  * wide modal #PhoshSystemPrompt dialogs
  */
 static GcrSystemPrompter *_prompter;
-static ulong owner_id;
+static gulong owner_id;
 static gboolean registered_prompter;
 static gboolean acquired_prompter;