Move BSSMAP_MSG_* defines to header file

This will be reused be several tests related to TS 48.008 and TS 29.205
in follow-up commits.

Change-Id: I4d8cc05b8df8e70c1f6257e53ae3acec7901681f
diff --git a/include/osmocom/gsm/gsm0808.h b/include/osmocom/gsm/gsm0808.h
index 9b19d69..298b3e4 100644
--- a/include/osmocom/gsm/gsm0808.h
+++ b/include/osmocom/gsm/gsm0808.h
@@ -29,6 +29,9 @@
 #include <osmocom/gsm/gsm23003.h>
 #include <osmocom/core/utils.h>
 
+#define BSSMAP_MSG_SIZE 512
+#define BSSMAP_MSG_HEADROOM 128
+
 struct sockaddr_storage;
 
 struct msgb;