Use system include <foo.h> notation for libosmocore headers

the curent local copies of libosmocore headers + source is a temporary
hack anyway. We should instead rely on a system-wide install of
libosmocore cross-compiled for arm-none-eabi.  But leave that as a
second (later) step beyond this patch.

Change-Id: Ia63fd842d45a2b404233b4326050e7eda0604cf0
diff --git a/firmware/libcommon/source/usb_buf.c b/firmware/libcommon/source/usb_buf.c
index ddeb43b..848fd24 100644
--- a/firmware/libcommon/source/usb_buf.c
+++ b/firmware/libcommon/source/usb_buf.c
@@ -2,8 +2,8 @@
 #include "trace.h"
 #include "usb_buf.h"
 
-#include "osmocom/core/linuxlist.h"
-#include "osmocom/core/msgb.h"
+#include <osmocom/core/linuxlist.h>
+#include <osmocom/core/msgb.h>
 #include <errno.h>
 
 #define USB_ALLOC_SIZE	280