[Silabs] [WiFi] Build fix for the wifi devices when using 91x bluetoo…

…th (#28809)

* adding the header file for the RS911x ble in ncp mode

* restyling the PR

src/platform/silabs/rs911x/wfx_sl_ble_init.c

@@ -22,6 +22,7 @@
  */
 #include "wfx_sl_ble_init.h"
 #include "ble_config.h"
+#include "cmsis_os2.h"
 #include "silabs_utils.h"
 // Global Variables
 rsi_ble_t att_list;