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

Change-Id: I60f5d4982cd96ab29f9924ec11b46bf56fbfc346
diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index e6e55a7..c11bf8d 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -19,7 +19,7 @@
  */
 #pragma once
 
-#include <sys/socket.h>
+struct sockaddr_storage;
 
 #include <osmocom/gsm/protocol/gsm_08_08.h>