gtp-kernel: don't #include libmnl headers

* we don't check for libmnl via pkg-config in configure.ac
* we don't add libmnl include path to CFLAGS

As a result, we cannot #include related files.

libmnl is completely encapsulated by libgtpnl.  It even
includes a forward-declaration of 'struct mnl_socket'.

Change-Id: I0af869cc3c8e30b69d73a4985c56ef7743565e95

contrib/osmo-ggsn.spec.in

@@ -30,7 +30,6 @@ BuildRequires:  pkgconfig >= 0.20
 BuildRequires:  systemd-rpm-macros
 %endif
 BuildRequires:  pkgconfig(libgtpnl) >= 1.0.0
-BuildRequires:  pkgconfig(libmnl) >= 1.0.3
 BuildRequires:  pkgconfig(libosmocore) >= 1.1.0
 BuildRequires:  pkgconfig(libosmoctrl) >= 1.1.0
 BuildRequires:  pkgconfig(libosmovty) >= 1.1.0

lib/gtp-kernel.c

@@ -18,7 +18,6 @@
 
 #include <libgtpnl/gtp.h>
 #include <libgtpnl/gtpnl.h>
-#include <libmnl/libmnl.h>
 
 #include <errno.h>