Add missing BuildRequires on gcc

make: gcc: Command not found

References: https://bugzilla.redhat.com/show_bug.cgi?id=1604616
Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>

libnatpmp.spec

@@ -6,6 +6,7 @@ License:        LGPLv2+
 URL:            http://miniupnp.free.fr/libnatpmp.html
 Source0:        http://miniupnp.free.fr/files/%{name}-%{version}.tar.gz
 
+BuildRequires:  gcc
 %description
 libnatpmp is an attempt to make a portable and fully compliant implementation
 of the protocol for the client side. It is based on non blocking sockets and