gsm0808 + ipa: fix compilation on systems without sys/socket.h

Change-Id: I60f5d4982cd96ab29f9924ec11b46bf56fbfc346
diff --git a/include/osmocom/gsm/gsm0808.h b/include/osmocom/gsm/gsm0808.h
index a9f610a..9be229c 100644
--- a/include/osmocom/gsm/gsm0808.h
+++ b/include/osmocom/gsm/gsm0808.h
@@ -21,7 +21,7 @@
 
 #include "tlv.h"
 #include <osmocom/gsm/protocol/gsm_08_08.h>
-#include <sys/socket.h>
+struct sockaddr_storage;
 
 struct msgb;