change #include "../hello.h" to "hello.h"

Before we can -I the top-level src/ directory alone, references to
"hello.h" must be updated.  This is an internal header, so it does
not need angle brackets.

src/target/testee.c

@@ -23,7 +23,7 @@
 #include "target.h"
 #include "target_type.h"
 #include <helper/log.h>
-#include "../hello.h"
+#include "hello.h"
 
 static const struct command_registration testee_command_handlers[] = {
 	{