lwip: add compatibility headers for sntp.h and esp_ping.h

Before LwIP was moved into a submodule, sntp and esp_ping were
available as the following includes:

#include "apps/sntp/sntp.h"
#include "esp_ping.h"

This commit makes header files available at the old location, and adds
a deprecation warning for sntp.h.

Closes https://github.com/espressif/esp-idf/issues/2392

components/lwip/apps/ping/esp_ping.c

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 #include <string.h>
-#include "ping/esp_ping.h"
+#include "esp_ping.h"
 
 #include "lwip/ip_addr.h"
 

components/lwip/apps/ping/ping.c

@@ -58,7 +58,7 @@
 #endif /* PING_USE_SOCKETS */
 
 #ifdef ESP_PING
-#include "ping/esp_ping.h"
+#include "esp_ping.h"
 #include "lwip/ip_addr.h"
 #endif
 /**