enlarge gsm0808 msgb headroom

It should be large enough to prepend a struct osmo_scu_prim to pass down an
SCCP stack (see libosmo-sccp). 264 should suffice, but pick the next larger
power-of-two instead.

In osmo-msc, I would like to prepend an osmo_prim to the msgb created by
gsm0808 API, but turns out the headroom is too small:

    msgb(0x61700001b660): Not enough headroom msgb_push (126 < 264)

Instead of always copying a msgb that has just that instant been created, it
makes more sense to allocate sufficient headroom in the first place.

Change-Id: I95729991eb49555f8bba60c5dc916131b03b6cf2

include/osmocom/gsm/gsm0808.h

@@ -29,8 +29,8 @@
 #include <osmocom/gsm/gsm23003.h>
 #include <osmocom/core/utils.h>
 
-#define BSSMAP_MSG_SIZE 512
-#define BSSMAP_MSG_HEADROOM 128
+#define BSSMAP_MSG_SIZE 1024
+#define BSSMAP_MSG_HEADROOM 512
 
 struct sockaddr_storage;