libpayload: Export usbhid_getmodifiers

Add a new method to retrieve active usb keyboard modifiers.

Change-Id: Ief6679ce782b58b9ced207f4f27504fb2a517b76
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/18602
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/libpayload.h

@@ -141,6 +141,7 @@ int usb_initialize(void);
 int usb_exit (void);
 int usbhid_havechar(void);
 int usbhid_getchar(void);
+int usbhid_getmodifiers(void);
 /** @} */
 
 /**