Moving grs_sndcp.h header file to include

For some reason gprs_sndcp.h is located in src/gprs. This commit moves
gprs_sndcp.h to include/openbsc and fixes the include path in
gprs_sndcp.c and gprs_sndcp_vty.c

Change-Id: If4e4f1252c81d7907c1b4d738c982bb172b128c9
diff --git a/openbsc/src/gprs/gprs_sndcp_vty.c b/openbsc/src/gprs/gprs_sndcp_vty.c
index deeef07..430881f 100644
--- a/openbsc/src/gprs/gprs_sndcp_vty.c
+++ b/openbsc/src/gprs/gprs_sndcp_vty.c
@@ -35,8 +35,7 @@
 #include <openbsc/debug.h>
 #include <openbsc/signal.h>
 #include <openbsc/gprs_llc.h>
-
-#include "gprs_sndcp.h"
+#include <openbsc/gprs_sndcp.h>
 
 #include <osmocom/vty/vty.h>
 #include <osmocom/vty/command.h>