errno: Avoid including strings in SPL

At present the header file defines this function in SPL but the file may
not actually be built. This causes a build error if the option is enabled.

Fix the condition in the header file.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/errno.h

@@ -25,7 +25,7 @@ extern int errno __errno_asm_label;
  * Return:	string describing the error. If CONFIG_ERRNO_STR is not
  *		defined an empty string is returned.
  */
-#ifdef CONFIG_ERRNO_STR
+#if CONFIG_IS_ENABLED(ERRNO_STR)
 const char *errno_str(int errno);
 #else
 static const char error_message[] = "";