Add libxslt as build requires for mingw RPMs

The libxslt package is needed since:

  commit 94d2d642
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Wed Jul 26 17:40:44 2017 +0100

    docs: make xmllint & xsltproc compulsory

The native RPM had it already, but mingw build was missing it.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

mingw-libvirt.spec.in

@@ -60,6 +60,7 @@ BuildRequires:  mingw64-dlfcn
 BuildRequires:  pkgconfig
 # Need native version for msgfmt
 BuildRequires:  gettext
+BuildRequires:  libxslt
 BuildRequires:  python
 %if 0%{?fedora} >= 27
 BuildRequires:  perl-interpreter