libvhost-user: Change dev->postcopy_ufd assignment to make it C90 compliant

libvhost-user: Change dev->postcopy_ufd assignment to make it C90 compliant

The assignment of dev->postcopy_ufd can be moved into an else clause and
then the code becomes C90 compliant.

  CC       libvhost-user.o
libvhost-user.c: In function ‘vu_set_postcopy_advise’:
libvhost-user.c:1625:5: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
 1625 |     struct uffdio_api api_struct;
      |     ^~~~~~

Understandable, it might be desired to avoid else clauses, but in this
case it seems clear enough and frankly the dev->postcopy_ufd is only
assigned once.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <74db52afb1203c4580ffc7fa462b4b2ba260a353.1671741278.git.marcel@holtmann.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>