setup.py: fix out of tree build

Fixes: commit 1cc41b1c9 ("pylibfdt: Add packaging metadata")

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220103073855.1468799-3-marcandre.lureau@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

setup.py

@@ -17,7 +17,7 @@ import sys
 
 srcdir = os.path.dirname(__file__)
 
-with open("README", "r") as fh:
+with open(os.path.join(srcdir, "README"), "r") as fh:
     long_description = fh.read()
 
 def get_top_builddir():