spec: Explicitly require libgcrypt-devel

Our configure.ac says:

    Not all versions of gnutls include -lgcrypt, and so we add
    it explicitly for the calls to gcry_control/check_version

Thus we cannot rely on gnutls-devel to bring grcypt-devel as a
dependency.

libvirt.spec.in

@@ -418,6 +418,7 @@ BuildRequires: readline-devel
 BuildRequires: ncurses-devel
 BuildRequires: gettext
 BuildRequires: libtasn1-devel
+BuildRequires: libgcrypt-devel
 BuildRequires: gnutls-devel
 BuildRequires: libattr-devel
 %if %{with_libvirtd}