usb: early: xhci-dbc: Supply missing 'xhci-dbgp.h' headerfile

If the header file containing a function's prototype isn't included by
the sourcefile containing the associated function, the build system
complains of missing prototypes.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/early/ehci-dbgp.c: In function ‘early_dbgp_write’:
 drivers/usb/early/ehci-dbgp.c:915:13: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
 915 | int chunk, ret;
 | ^~~
 drivers/usb/early/xhci-dbc.c:600:12: warning: no previous prototype for ‘early_xdbc_parse_parameter’ [-Wmissing-prototypes]
 600 | int __init early_xdbc_parse_parameter(char *s)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/usb/early/xhci-dbc.c:653:12: warning: no previous prototype for ‘early_xdbc_setup_hardware’ [-Wmissing-prototypes]
 653 | int __init early_xdbc_setup_hardware(void)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/usb/early/xhci-dbc.c:910:13: warning: no previous prototype for ‘early_xdbc_register_console’ [-Wmissing-prototypes]
 910 | void __init early_xdbc_register_console(void)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20200706133341.476881-27-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/early/xhci-dbc.c

@@ -21,6 +21,7 @@
 #include <linux/module.h>
 #include <linux/delay.h>
 #include <linux/kthread.h>
+#include <linux/usb/xhci-dbgp.h>
 
 #include "../host/xhci.h"
 #include "xhci-dbc.h"