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/generator/sections/gen-section.c b/generator/sections/gen-section.c
index 56b6d84..a5ae40b 100644
--- a/generator/sections/gen-section.c
+++ b/generator/sections/gen-section.c
@@ -3,7 +3,7 @@
  *
  * Author: Lawrence.Tang@arm.com
  **/
-#include "gen-section.h"
+#include <libcper/generator/sections/gen-section.h>
 
 CPER_GENERATOR_DEFINITION generator_definitions[] = {
 	{ &gEfiProcessorGenericErrorSectionGuid, "generic",