gdb: add declaration for _initialize_gdbarch in gdbarch.sh

In commit

  gdb: add back declarations for _initialize functions
  6c2659886f

I wrongfully edited gdbarch.c, instead of editing gdbarch.sh and
re-generating gdbarch.c.  This patch fixes gdbarch.sh to add a
declaration for _initialize_gdbarch.  gdbarch.c is not changed, as the
output of gdbarch.sh now matches the current state of gdbarch.c.

gdb/ChangeLog:

	* gdbarch.sh: Add declaration for _initialize_gdbarch.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-01-21  Simon Marchi  <simon.marchi@efficios.com>
+
+	* gdbarch.sh: Add declaration for _initialize_gdbarch.
+
 2020-01-21  Simon Marchi  <simon.marchi@efficios.com>
 
 	* remote-sim.c (check_for_duplicate_sim_descriptor): Remove.

gdb/gdbarch.sh

@@ -2600,8 +2600,9 @@ target_gdbarch (void)
   return current_inferior ()->gdbarch;
 }
 
+void _initialize_gdbarch ();
 void
-_initialize_gdbarch (void)
+_initialize_gdbarch ()
 {
   add_setshow_zuinteger_cmd ("arch", class_maintenance, &gdbarch_debug, _("\\
 Set architecture debugging."), _("\\