Restore FFI-ability of struct mg_str

mgos_dlsym needs to include mg_str for ffi_exports to be able to use it

PUBLISHED_FROM=a2dd167c8e8f9e9b7316a856f19a4603080c3632

Author: Deomid Ryabkov

src/mgos_bt.c

@@ -15,12 +15,13 @@
  * limitations under the License.
  */
 
+#include <stdio.h>
+#include <string.h>
+
 #include "mgos_bt.h"
 #include "mgos_bt_gattc.h"
 #include "mgos_system.h"
 
-#include <stdio.h>
-
 const char *mgos_bt_addr_to_str(const struct mgos_bt_addr *addr, char *out) {
   sprintf(out, "%02x:%02x:%02x:%02x:%02x:%02x", addr->addr[0], addr->addr[1],
           addr->addr[2], addr->addr[3], addr->addr[4], addr->addr[5]);