restore inc/qcbor.h for backwards compatibility (#181)

A long time ago there was just inc/qcbor.h. This file was moved to inc/qcbor/qcbor.h to facilitate installation in /usr/local/include. inc/qcbor.h was retained for a while, but got lost in a big merge in 2020. This PR restores it. It simply includes inc/qcbor/qcbor.h.
diff --git a/inc/qcbor.h b/inc/qcbor.h
new file mode 100644
index 0000000..e5b23e6
--- /dev/null
+++ b/inc/qcbor.h
@@ -0,0 +1 @@
+#include "qcbor/qcbor.h"