Include necessary header

Clang complains about not being able to find the printf
implementation without this include

libsel4test/include/sel4test/macros.h

diff --git a/libsel4test/include/sel4test/macros.h b/libsel4test/include/sel4test/macros.h
index 8b3dfa2..e021983 100644
--- a/libsel4test/include/sel4test/macros.h
+++ b/libsel4test/include/sel4test/macros.h
@@ -15,6 +15,7 @@
 /* Include Kconfig variables. */
 #include <autoconf.h>
 #include <sel4test/gen_config.h>
+#include <sel4test/testutil.h>
 
 #define SEL4TEST_PRINT_BUFFER 200