Change include style to use system includes

The libcper header files in `libcper` are installed to
`usr/include/libcper`. Use that system includes in `libcper` source
instead of using the project includes.

Change-Id: I596edc2c754dae4829844f535a1e34caa246fb43
Signed-off-by: Thu Nguyen <thu@os.amperecomputing.com>
diff --git a/tests/ir-tests.cpp b/tests/ir-tests.cpp
index 0146cac..5041a23 100644
--- a/tests/ir-tests.cpp
+++ b/tests/ir-tests.cpp
@@ -8,11 +8,11 @@
 #include "gtest/gtest.h"
 #include "test-utils.hpp"
 #include <json.h>
-#include "../cper-parse.h"
-#include "../json-schema.h"
-#include "../generator/cper-generate.h"
-#include "../sections/cper-section.h"
-#include "../generator/sections/gen-section.h"
+#include <libcper/cper-parse.h>
+#include <libcper/json-schema.h>
+#include <libcper/generator/cper-generate.h>
+#include <libcper/sections/cper-section.h>
+#include <libcper/generator/sections/gen-section.h>
 
 /*
 * Test templates.