spec: mingw: add deps for genprotocol.pl usage

As of 0985a959 we no longer distribute generated files. mingw
needs to call genprotocol.pl itself, which means it needs rpcgen
and cpp installed.
Signed-off-by: NCole Robinson <crobinso@redhat.com>

mingw-libvirt.spec.in

@@ -89,6 +89,10 @@ BuildRequires: mingw64-libssh2
 BuildRequires: mingw32-curl
 BuildRequires: mingw64-curl
 %endif
+BuildRequires: cpp
+%if 0%{?fedora} || 0%{?rhel} > 7
+BuildRequires: rpcgen
+%endif
 
 BuildArch:      noarch