Add forgotten USB header

cores/rp2040/RP2040USB.h

+/*
+ * Shared USB for the Raspberry Pi Pico RP2040
+ * Allows for multiple endpoints to share the USB controller
+ *
+ * Copyright (c) 2021 Earle F. Philhower, III <earlephilhower@yahoo.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#include "pico/mutex.h"
+
+// Weak function definitions for each type of endpoint
+extern void __USBInstallSerial() __attribute__((weak));
+extern void __USBInstallKeyboard() __attribute__((weak));
+extern void __USBInstallMouse() __attribute__((weak));
+extern void __USBInstallMIDI() __attribute__((weak));
+
+// Big, global USB mutex, shared with all USB devices to make sure we don't
+// have multiple cores updating the TUSB state in parallel
+extern mutex_t __usb_mutex;
+
+// HID report ID inquiry (report ID will vary depending on the number/type of other HID)
+int __USBGetKeyboardReportID();
+int __USBGetMouseReportID();
+
+// Called by main() to init the USB HW/SW.
+void __USBStart();