hw/registerfields: add missing include

hw/registerfields: add missing include

This allows to use this header in qtests.

This fixes:
    CC      tests/test.o
  include/hw/registerfields.h:32:41: error: implicit declaration of function ‘MAKE_64BIT_MASK’ [-Werror=implicit-function-declaration]
              MAKE_64BIT_MASK(shift, length)};
              ^
  include/hw/registerfields.h:39:5: error: implicit declaration of function ‘extract64’; [-Werror=implicit-function-declaration]
       extract64((storage), R_ ## reg ## _ ## field ## _SHIFT,
       ^

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>