Normalize includes relative to source root

For unit test cases it is cleaner to have include file paths relative to
source root (e.g. #include attn/attn_handler.hpp).

Signed-off-by: Ben Tyner <ben.tyner@ibm.com>
Change-Id: Id718e57e9ba8da4c2ff3bb11b1c7a61537da538a
diff --git a/attn/bp_handler.cpp b/attn/bp_handler.cpp
index 72e03f2..e25a243 100644
--- a/attn/bp_handler.cpp
+++ b/attn/bp_handler.cpp
@@ -1,4 +1,4 @@
-#include <attn_logging.hpp>
+#include <attn/attn_logging.hpp>
 #include <sdbusplus/bus.hpp>
 
 namespace attn