Our ticket-based spinlock structures rely on a definition of u16, so
include linux/types.h explicitly to ensure the thing compiles.

Found by a module build failure in -next:

  arch/arm64/include/asm/spinlock_types.h:27:2: error: unknown type name 'u16'
  arch/arm64/include/asm/spinlock_types.h:28:2: error: unknown type name 'u16'
  arch/arm64/include/asm/spinlock_types.h:33:13: error: expected declaration specifiers or '...' before numeric constant
  include/linux/spinlock_types.h:21:2: error: unknown type name 'arch_spinlock_t'
  arch/arm64/include/asm/spinlock.h:34:35: error: unknown type name 'arch_spinlock_t'
  arch/arm64/include/asm/spinlock.h:65:37: error: unknown type name 'arch_spinlock_t'
Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Will Deacon <will.deacon@arm.com>