autotools: Install headers

Use the _HEADERS automake variables to install headers appropriately.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/libmctp-astlpc.h b/libmctp-astlpc.h
index 4592e75..4bffd93 100644
--- a/libmctp-astlpc.h
+++ b/libmctp-astlpc.h
@@ -7,7 +7,7 @@
 extern "C" {
 #endif
 
-#include "libmctp.h"
+#include <libmctp.h>
 
 struct mctp_binding_astlpc;