tile: add init.h to usb.c to avoid compile failure

tile: add init.h to usb.c to avoid compile failure

Pending header cleanups will reveal this file is using the
init.h content implicitly with the following fail:

arch/tile/kernel/usb.c:69:1: warning: data definition has no type or storage class [enabled by default]
arch/tile/kernel/usb.c:69:1: error: type defaults to 'int' in declaration of 'arch_initcall'
arch/tile/kernel/usb.c:69:1: warning: parameter names (without types) in function declaration [enabled by default]
arch/tile/kernel/usb.c:62:19: warning: 'tilegx_usb_init' defined but not used

Explicitly add init.h to get arch_initcall and avoid this.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Acked-by: Chris Metcalf <cmetcalf@ezchip.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>