libvirt:spec: Require gcc for build

According to latest discussion on fedora devel list [1] gcc will
be removed from default buildroot and packages requiring it must
have explicit build dependency,

1: https://lists.fedoraproject.org/archives/list/devel-announce@lists.fedoraproject.org/thread/IJFYI5Q2BYZKIGDFS2WLOBDUSEGWHIKV/Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>

libvirt.spec.in

@@ -298,6 +298,7 @@ BuildRequires: gettext-devel
 BuildRequires: libtool
 BuildRequires: /usr/bin/pod2man
 %endif
+BuildRequires: gcc
 BuildRequires: git
 %if 0%{?fedora} >= 27 || 0%{?rhel} > 7
 BuildRequires: perl-interpreter