kgdboc: Fix warning with module build

After 2dd4531 ("kgdboc: Fix restrict error"), kgdboc_option_setup is
now only used when built in, resulting in a warning when compiled as a
module:

drivers/tty/serial/kgdboc.c:134:12: warning: 'kgdboc_option_setup' defined but not used [-Wunused-function]
 static int kgdboc_option_setup(char *opt)
            ^~~~~~~~~~~~~~~~~~~

Move the function under the appropriate ifdef for builtin only.

Fixes: 2dd4531 ("kgdboc: Fix restrict error")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/kgdboc.c

@@ -131,24 +131,6 @@ static void kgdboc_unregister_kbd(void)
 #define kgdboc_restore_input()
 #endif /* ! CONFIG_KDB_KEYBOARD */
 
-static int kgdboc_option_setup(char *opt)
-{
-	if (!opt) {
-		pr_err("config string not provided\n");
-		return -EINVAL;
-	}
-
-	if (strlen(opt) >= MAX_CONFIG_LEN) {
-		pr_err("config string too long\n");
-		return -ENOSPC;
-	}
-	strcpy(config, opt);
-
-	return 0;
-}
-
-__setup("kgdboc=", kgdboc_option_setup);
-
 static void cleanup_kgdboc(void)
 {
 	if (kgdb_unregister_nmi_console())
@@ -316,6 +298,25 @@ static struct kgdb_io kgdboc_io_ops = {
 };
 
 #ifdef CONFIG_KGDB_SERIAL_CONSOLE
+static int kgdboc_option_setup(char *opt)
+{
+	if (!opt) {
+		pr_err("config string not provided\n");
+		return -EINVAL;
+	}
+
+	if (strlen(opt) >= MAX_CONFIG_LEN) {
+		pr_err("config string too long\n");
+		return -ENOSPC;
+	}
+	strcpy(config, opt);
+
+	return 0;
+}
+
+__setup("kgdboc=", kgdboc_option_setup);
+
+
 /* This is only available if kgdboc is a built in for early debugging */
 static int __init kgdboc_early_init(char *opt)
 {