Don't install osmo-ggsn-kernel-gtp.cfg to /etc/osmocom/

This is an auxiliary example config file, which should not be installed
to /etc/ and hence not be in OSMOCONF_FILES

This fixes the following rpm packaging error:
[  149s] error: Installed (but unpackaged) file(s) found:
[  149s]    /etc/osmocom/osmo-ggsn-kernel-gtp.cfg

Change-Id: If118ed26491a1edda83eda7f95479e165ca4c150
Fixes: I6fbe8a8e55bad41532e9aed3cf71ebebffdcee52

doc/examples/Makefile.am

@@ -1,5 +1,4 @@
 OSMOCONF_FILES = \
-	osmo-ggsn-kernel-gtp.cfg \
 	osmo-ggsn.cfg \
 	sgsnemu.conf \
 	$(NULL)