device/include: Fix potential build error

Add include guard for usbc_mux.h

BUG=none
BRANCH=none
TEST=Build Pass

Signed-off-by: Alan Huang <alan-huang@quanta.corp-partner.google.com>
Change-Id: I47988edee84d17f0a15cfda1ac6f0187326bd331
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58832
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/include/device/usbc_mux.h

@@ -1,5 +1,8 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#ifndef __USBC_MUX_H__
+#define __USBC_MUX_H__
+
 /* struct to hold all USB-C mux related variables */
 struct usbc_mux_info {
 	bool dp; /* DP connected */
@@ -67,3 +70,5 @@ struct usbc_ops {
 };
 
 const struct usbc_ops *usbc_get_ops(void);
+
+#endif /* __USBC_MUX_H__ */