gprs_gmm: introduce a GMM Attach Request FSM

The old GMM Attach Request handling used a recursive function
which can not handle certain states and is quite complex and hard to
extend.

The new FSM handles such request in a FSM and can be called multiple
times.

Change-Id: I58b9c17be9776a03bb2a5b21e99135cfefc8c912
diff --git a/include/osmocom/sgsn/Makefile.am b/include/osmocom/sgsn/Makefile.am
index 269cebc..3b563c4 100644
--- a/include/osmocom/sgsn/Makefile.am
+++ b/include/osmocom/sgsn/Makefile.am
@@ -5,6 +5,7 @@
 	gb_proxy.h \
 	gprs_gb_parse.h \
 	gprs_gmm.h \
+	gprs_gmm_attach.h \
 	gprs_llc.h \
 	gprs_llc_xid.h \
 	gprs_sgsn.h \