move to libcommon-cs: net init 2: move bsc_network_init decl to new .h

bsc_network_init() is more fit to live in a BSC specific header, move it to new
common_bsc.h. It will probably also absorb the BSC-specific part of gsm_network
in the future.

Adjust header includes across the board. Particularly, fix abis_nm.h by
explicitly including gsm_data.h: it so far relied on other headers to do that,
which now is no longer always given.

Change-Id: I9edfb1e748bb1cb484fadd48b0406f5b3098e89b
diff --git a/openbsc/tests/channel/channel_test.c b/openbsc/tests/channel/channel_test.c
index 0c730a2..351bb5a 100644
--- a/openbsc/tests/channel/channel_test.c
+++ b/openbsc/tests/channel/channel_test.c
@@ -25,6 +25,7 @@
 #include <osmocom/core/application.h>
 #include <osmocom/core/select.h>
 
+#include <openbsc/common_bsc.h>
 #include <openbsc/abis_rsl.h>
 #include <openbsc/debug.h>
 #include <openbsc/gsm_subscriber.h>