[-Wunused-variable] - pretend to use

Looks like a SWIG bug, creating an extra variable that it doesn't use

bindings/python/gnucash_core.c: In function ‘_wrap_xaccAccountGetReconcileLastDate’:
bindings/python/gnucash_core.c:10096:10: warning: unused variable ‘secs20’ [-Wunused-variable]
10096 |   time64 secs20 ;
      |          ^~~~~~
bindings/python/gnucash_core.c: In function ‘_wrap_xaccAccountGetReconcilePostponeDate’:
bindings/python/gnucash_core.c:10298:10: warning: unused variable ‘secs20’ [-Wunused-variable]
10298 |   time64 secs20 ;
      |          ^~~~~~
bindings/python/gnucash_core.c: In function ‘_wrap_qof_query_date_predicate_get_date’:
bindings/python/gnucash_core.c:20237:10: warning: unused variable ‘secs20’ [-Wunused-variable]
20237 |   time64 secs20 ;
      |          ^~~~~~

bindings/python/time64.i

@@ -137,6 +137,10 @@
   PyObject *tp;
   struct tm t;
 
+  // SWIG 4.02 (maybe others?) generates a redundant variable "time64 secs20"
+  // This line avoids the unused-variable warning
+  (void) secs;
+
   // directly access return value (result) of function
   // only return datetime if TRUE
   if(result) {