oap_message.h: Remove dependency to openbsc include

This is a first step to moving oap_messages.h to libosmocore
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>