Add missing BuildRequires on gcc

make: cc: Command not found

References: https://bugzilla.redhat.com/show_bug.cgi?id=1606739
Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>

xl2tpd.spec

@@ -19,6 +19,7 @@ Patch4: xl2tpd-1.3.8-kernelmode.patch
 Requires: ppp >= 2.4.5-18, kmod(l2tp_ppp.ko)
 # If you want to authenticate against a Microsoft PDC/Active Directory
 # Requires: samba-winbind
+BuildRequires:  gcc
 BuildRequires: libpcap-devel
 BuildRequires: systemd-units
 BuildRequires: openssl-devel