vl: Add missing "hw/boards.h" include

vl.c calls machine_usb() declared in "hw/boards.h". Include it.

This fixes (when modifying unrelated headers):

  vl.c:1283:10: error: implicit declaration of function 'machine_usb' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
      if (!machine_usb(current_machine)) {
           ^
  vl.c:1283:10: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
  vl.c:1283:22: error: use of undeclared identifier 'current_machine'
      if (!machine_usb(current_machine)) {
                       ^

Acked-by: John Snow <jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200228114649.12818-2-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>