tests: custom_lib: Fix include paths

Align to the new standard that prefixes zephyr/.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

Author: carlescufi

tests/lib/custom_lib/src/main.c

@@ -11,8 +11,8 @@
  * library works correctly.
  */
 
-#include <zephyr.h>
-#include <ztest.h>
+#include <zephyr/kernel.h>
+#include <zephyr/ztest.h>
 #include <custom_lib/custom_lib.h>
 
 static void test_get_value(void)