libvhost-user: Declare uffdio_register early to make it C90 compliant

libvhost-user: Declare uffdio_register early to make it C90 compliant

When using libvhost-user source in an external project that wants to
comply with the C90 standard, it is best to declare variables before
code.

  CC       libvhost-user.o
libvhost-user.c: In function ‘generate_faults’:
libvhost-user.c:683:9: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
  683 |         struct uffdio_register reg_struct;
      |         ^~~~~~

In this case, it is also simple enough and doesn't cause any extra
ifdef additions.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <556c2d00c01fa134d13c0371d4014c90694c2943.1671741278.git.marcel@holtmann.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>