samples/net: Fix format warning in the HTTP client sample app

Fix a printk format warning found at the HTTP client sample app.

Jira: ZEP-1872
Change-Id: I9665e3e59595b383d6e809af51fe4cf3cd8f8bd8
Signed-off-by: Flavio Santes <[email protected]>
(cherry picked from commit 2b66787a9344c41fb807b4b680a57a7f2edf6689)

Author: 1010101001010101

samples/net/http_client/src/http_client.c

@@ -96,7 +96,7 @@ int http_send_request(struct http_client_ctx *http_ctx, const char *method,
 
 		rc = snprintk(content_len_str, sizeof(content_len_str),
 			      "\r\nContent-Length: %u\r\n\r\n",
-			      strlen(payload));
+			      (unsigned int)strlen(payload));
 		if (rc <= 0 || rc >= sizeof(content_len_str)) {
 			rc = -ENOMEM;
 			goto lb_exit;