gh-92356: Fix regression in ctypes function call overhead (#92357)

38f331d introduced a delayed initialization routine to set up
ctypes formattable (`_ctypes_init_fielddesc`), but inadvertently
removed setting the `initialization` flag to 1 to avoid initting
each time.

Author: mdboom

Misc/NEWS.d/next/Library/2022-05-05-19-25-09.gh-issue-92356.uvxWdu.rst

@@ -0,0 +1 @@
+Fixed a performance regression in ctypes function calls.

Modules/_ctypes/cfield.c

@@ -1602,6 +1602,7 @@ _ctypes_get_fielddesc(const char *fmt)
     struct fielddesc *table = formattable;
 
     if (!initialized) {
+        initialized = 1;
         _ctypes_init_fielddesc();
     }