Fix unused variable warning in wp_seed_src.c

Wrap g_seed_cb_registered declaration in #ifdef WC_RNG_SEED_CB since
the variable is only used when that macro is defined. This fixes the
-Werror build failure due to unused-variable warning.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ColtonWilley

src/wp_seed_src.c

@@ -71,7 +71,9 @@ static XFILE g_urandom_file = XBADFILE;
 /*
  * Flag indicating whether the seed callback has been registered.
  */
+#ifdef WC_RNG_SEED_CB
 static int g_seed_cb_registered = 0;
+#endif
 
 /**
  * wolfSSL seed callback that uses the cached /dev/urandom file.