fix size_t compile error

class/hid/usbh_hid.c

@@ -68,7 +68,7 @@ static void usbh_hid_devno_free(struct usbh_hid *priv)
     int devno = priv->minor;
 
     if (devno >= 0 && devno < 32) {
-        size_t size_t = usb_osal_enter_critical_section();
+        size_t flags = usb_osal_enter_critical_section();
         g_devinuse &= ~(1 << devno);
         usb_osal_leave_critical_section(flags);
     }

osal/usb_osal.h

@@ -24,6 +24,7 @@
 #define _USB_OSAL_H
 
 #include <stdint.h>
+#include <string.h>
 
 typedef void *usb_osal_thread_t;
 typedef void *usb_osal_sem_t;