Fix compiler warnings due to new checks in PostgreSQL 16

See the commit 0fe954c28584169938e5c0738cfaa9930ce77577 (Add
-Wshadow=compatible-local to the standard compilation flags) in PostgreSQL 16.

pg_wait_sampling.c: In function ‘pg_wait_sampling_get_current’:
pg_wait_sampling.c:454:42: warning: declaration of ‘params’ shadows a previous
local [-Wshadow=compatible-local]
  454 |                 WaitCurrentContext      *params;
      |                                          ^~~~~~
pg_wait_sampling.c:446:34: note: shadowed declaration is here
  446 |         WaitCurrentContext      *params;
      |                                  ^~~~~~

Author: Marina Polyakova

pg_wait_sampling.c

@@ -451,7 +451,6 @@ pg_wait_sampling_get_current(PG_FUNCTION_ARGS)
 	{
 		MemoryContext		oldcontext;
 		TupleDesc			tupdesc;
-		WaitCurrentContext 	*params;
 
 		funcctx = SRF_FIRSTCALL_INIT();