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/json-schema.c b/json-schema.c
index dbbffbc..9eb60be 100644
--- a/json-schema.c
+++ b/json-schema.c
@@ -12,8 +12,8 @@
 #include <limits.h>
 #include <stdarg.h>
 #include <json.h>
-#include "json-schema.h"
-#include "BaseTypes.h"
+#include <libcper/json-schema.h>
+#include <libcper/BaseTypes.h>
 
 //Field definitions.
 int json_validator_debug = 0;