osmo-bsc_nat: Don't link against libosmonetif

This fixes the following dpkg-shlibdeps warning:

Change-Id: I31af5fb8b52ef1fd5effb139d9cdea1ebe9a41b4
dpkg-shlibdeps: warning: package could avoid a useless dependency if debian/osmo-bsc/usr/bin/osmo-bsc_nat was not linked against libosmonetif.so.4 (it uses none of the library's symbols)

src/osmo-bsc_nat/Makefile.am

@@ -12,7 +12,6 @@ AM_CFLAGS = \
 	$(LIBOSMOCTRL_CFLAGS) \
 	$(LIBOSMOSCCP_CFLAGS) \
 	$(LIBOSMOABIS_CFLAGS) \
-	$(LIBOSMONETIF_CFLAGS) \
 	$(LIBOSMOLEGACYMGCP_CFLAGS) \
 	$(LIBCRYPTO_CFLAGS) \
 	$(COVERAGE_CFLAGS) \
@@ -52,7 +51,6 @@ osmo_bsc_nat_LDADD = \
 	$(LIBOSMOVTY_LIBS) \
 	$(LIBOSMOCTRL_LIBS) \
 	$(LIBOSMOABIS_LIBS) \
-	$(LIBOSMONETIF_LIBS) \
 	$(LIBOSMOSIGTRAN_LIBS) \
 	$(LIBOSMOLEGACYMGCP_LIBS) \
 	$(LIBCRYPTO_LIBS) \