Fix link error for test_radixtree module on Windows

Add PGDLLIMPORT to pg_popcount32/64. In passing, fix a typo.

Diagnosis by Masahiko Sawada, patch by David Rowley

Per buildfarm members drongo and fairywren

Discussion: https://postgr.es/m/CAD21AoAMm1mQd%3Dw4PrfrKK%3DOMP8j8%3D7ntJRPF8%2B%3D10iUuvwiCA%40mail.gmail.com
Discussion: https://postgr.es/m/CAApHDvov7724UrD1Ug0D1eV%2B9Pd_x5VEQmw-6HVG9w1WdCxXPA%40mail.gmail.com

src/include/port/pg_bitutils.h

@@ -300,8 +300,8 @@ pg_ceil_log2_64(uint64 num)
 
 #ifdef TRY_POPCNT_FAST
 /* Attempt to use the POPCNT instruction, but perform a runtime check first */
-extern int	(*pg_popcount32) (uint32 word);
-extern int	(*pg_popcount64) (uint64 word);
+extern PGDLLIMPORT int (*pg_popcount32) (uint32 word);
+extern PGDLLIMPORT int (*pg_popcount64) (uint64 word);
 
 #else
 /* Use a portable implementation -- no need for a function pointer. */

src/test/modules/test_radixtree/test_radixtree.c

@@ -1,7 +1,7 @@
 /*--------------------------------------------------------------------------
  *
  * test_radixtree.c
- *		Test module for adapive radix tree.
+ *		Test module for adaptive radix tree.
  *
  * Copyright (c) 2024, PostgreSQL Global Development Group
  *