Fix missing error codes when NO_SOCK and USER_IO are in use

Functions in `wolfio.c` unconditionally reference SOCKET_* error
codes, even if `errno.h` is conditionally includes in wolfio.h.

When compiled with at least WOLFSSL_NO_SOCK + WOLFSSO_USER_IO
`wolfio.h` decides not to include `errno.h` and therefore SOCKET_* symbols
are missing.

Instead of changing "decision tree" in `wolfio.h` and
risking breaking it on platforms I couldn't test, use "last resort"
include of 'errno.h' in the wolfio.c if configure process detected
its presence.

src/wolfio.c

@@ -30,6 +30,10 @@
 
 #include <wolfssl/wolfcrypt/settings.h>
 
+#ifdef HAVE_ERRNO_H
+    #include <errno.h>
+#endif
+
 #ifndef WOLFCRYPT_ONLY
 
 #ifdef _WIN32_WCE