fix(mqtt_utils): Include stdint.h for intptr_t

mqtt_create_uri_from_string uses intptr_t but includes only string.h,
ctype.h and stdlib.h. It builds today because one of those reaches
stdint.h transitively on the toolchains in use; GCC 16 no longer does,
and the file stops compiling:

  mqtt_utils.c:36:10: error: unknown type name 'intptr_t'
     36 |     for (intptr_t i = 0; i < uri_len; i++, write_ptr++) {
        |          ^~~~~~~~

Include the header the type comes from rather than rely on another one
to bring it in.

Signed-off-by: Nebojša Cvetković <[email protected]>

lib/mqtt_utils/mqtt_utils.c

@@ -5,6 +5,7 @@
  */
 #include <string.h>
 #include <ctype.h>
+#include <stdint.h>
 #include <stdlib.h>
 #include "include/mqtt_utils.h"