oap_message.h: Remove dependency to openbsc include

This is a first step to moving oap_messages.h to libosmocore
diff --git a/openbsc/include/openbsc/oap_messages.h b/openbsc/include/openbsc/oap_messages.h
index b7f4cca..e76287b 100644
--- a/openbsc/include/openbsc/oap_messages.h
+++ b/openbsc/include/openbsc/oap_messages.h
@@ -22,8 +22,8 @@
 #pragma once
 
 #include <stdint.h>
+#include <osmocom/core/msgb.h>
 #include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
-#include <openbsc/gsm_data.h>
 
 /* Some numbers are out of sequence because (so far) they match gprs_gsup_iei.
  */
diff --git a/openbsc/src/gprs/oap.c b/openbsc/src/gprs/oap.c
index f5d7a95..eb0dd19 100644
--- a/openbsc/src/gprs/oap.c
+++ b/openbsc/src/gprs/oap.c
@@ -20,6 +20,8 @@
  *
  */
 
+#include <string.h>
+
 #include <osmocom/core/utils.h>
 #include <osmocom/crypt/auth.h>
 
diff --git a/openbsc/src/gprs/oap_messages.c b/openbsc/src/gprs/oap_messages.c
index d5750a6..ccb3c81 100644
--- a/openbsc/src/gprs/oap_messages.c
+++ b/openbsc/src/gprs/oap_messages.c
@@ -24,7 +24,6 @@
 #include <openbsc/oap_messages.h>
 
 #include <openbsc/debug.h>
-#include <openbsc/gprs_utils.h>
 
 #include <osmocom/gsm/tlv.h>
 #include <osmocom/core/msgb.h>
diff --git a/openbsc/tests/oap/oap_test.c b/openbsc/tests/oap/oap_test.c
index 9a2063b..4ee5d42 100644
--- a/openbsc/tests/oap/oap_test.c
+++ b/openbsc/tests/oap/oap_test.c
@@ -26,6 +26,7 @@
 #include <openbsc/oap_messages.h>
 
 #include <stdio.h>
+#include <string.h>
 
 static void test_oap_api(void)
 {