Fix libusb context null problem

Author: jiegec

drivers/libusb.h

@@ -2,7 +2,7 @@
 #define COMMON_H
 
 #include <libusb.h>
-static libusb_context *context = nullptr;
+extern libusb_context *context;
 
 #endif
 

main.cpp

@@ -2,9 +2,12 @@
 #include "mainwindow.h"
 #include <QApplication>
 
+libusb_context *context = nullptr;
+
 int main(int argc, char *argv[]) {
   auto rc = libusb_init(&context);
   Q_ASSERT(rc >= 0);
+  Q_ASSERT(context != nullptr);
 
   QCoreApplication::setOrganizationName("TUNA");
   QCoreApplication::setOrganizationDomain("tuna.tsinghua.edu.cn");