gmain: Include poll.h as early as possible

On AIX, the system poll.h redefines the names of struct members,
for example `#define events reqevents`. This means that accesses
to GPollFD will fail to compile if poll.h was included after
glib/gpoll.h.

We can't simply add `#include <poll.h>` in glib/gpoll.h, because
that wouldn't work on platforms where poll.h doesn't exist, and
GLib supports some platforms in that category.

Resolves: https://gitlab.gnome.org/GNOME/glib/-/issues/3500

glib/gmain.c

@@ -48,6 +48,15 @@
 #define G_MAIN_POLL_DEBUG
 #endif
 
+/* We need to include this as early as possible, because on some
+ * platforms like AIX, <poll.h> redefines the names we use for
+ * GPollFD struct members.
+ * See https://gitlab.gnome.org/GNOME/glib/-/issues/3500 */
+
+#ifdef HAVE_POLL_H
+#include <poll.h>
+#endif
+
 #ifdef G_OS_UNIX
 #include "glib-unix.h"
 #include <pthread.h>
@@ -69,10 +78,6 @@
 #include <errno.h>
 #include <string.h>
 
-#ifdef HAVE_POLL_H
-#include <poll.h>
-#endif
-
 #ifdef HAVE_PIDFD
 #include <sys/syscall.h>
 #include <sys/wait.h>