src/Makefile: keep private headers in <liburing/*.h>

It is not possible to install barrier.h and compat.h into the top-level
/usr/include directly since they are likely to conflict with other
software.  io_uring.h could be confused with the system's kernel header
file.

Put liburing headers into <liburing/*.h> so there is no chance of
conflicts or confusion.

Existing applications continue to build successfully since the location
of <liburing.h> is unchanged.  In-tree examples and tests require
modification because src/liburing.h is moved to src/include/liburing.h.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/test/io_uring_enter.c b/test/io_uring_enter.c
index b25afd5..c2030c1 100644
--- a/test/io_uring_enter.c
+++ b/test/io_uring_enter.c
@@ -22,8 +22,8 @@
 #include <sys/resource.h>
 #include <limits.h>
 #include <sys/time.h>
-#include "../src/liburing.h"
-#include "../src/barrier.h"
+#include "liburing.h"
+#include "liburing/barrier.h"
 
 #define IORING_MAX_ENTRIES 4096