[fix](trx-ft8): fix const qualifier warning in ft8_lib C code

Fix compiler warning about discarded const qualifier in strchr() call.
The result of strchr(const char*, ...) should be assigned to const char*,
not char*.

This resolves:
  warning: initialization discards 'const' qualifier from pointer target type

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

external/ft8_lib/ft8/message.c

@@ -214,7 +214,7 @@ ftx_message_rc_t ftx_message_encode_std(ftx_message_t* msg, ftx_callsign_hash_in
         }
     }
 
-    char* slash_de = strchr(call_de, '/');
+    const char* slash_de = strchr(call_de, '/');
     uint8_t icq = (uint8_t)equals(call_to, "CQ") || starts_with(call_to, "CQ ");
     if (slash_de && (slash_de - call_de >= 2) && icq && !(equals(slash_de, "/P") || equals(slash_de, "/R")))
     {