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/meson.build b/tests/meson.build
index abfc497..68eb1f3 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -21,7 +21,7 @@
 
 sources = ['ir-tests.cpp', 'test-utils.cpp', 'base64_test.cpp']
 
-test_include_dirs = ['.', '..']
+test_include_dirs = ['.', '../include']
 
 cper_tests = executable(
     'cper-tests',