Add dummy implementation when strerror() is missing

Use a simple empty string for platforms without an implementation.
diff --git a/lib/private-libwebsockets.h b/lib/private-libwebsockets.h
index e27c3c2..53aada5 100644
--- a/lib/private-libwebsockets.h
+++ b/lib/private-libwebsockets.h
@@ -105,6 +105,10 @@
 #define compatible_close(fd) close(fd);
 #endif
 
+#ifndef HAVE_STRERROR
+#define strerror(x) ""
+#endif
+
 #ifdef LWS_OPENSSL_SUPPORT
 #ifdef USE_CYASSL
 #include <cyassl/openssl/ssl.h>