target: rtt: include rtt.h

Let source file to include its file .h to validate the exported
prototypes.

Detected through 'sparse' tool.

Change-Id: I8ae2f8f1fdaea5683e157247463533b17237e464
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7602
Tested-by: jenkins

src/target/rtt.c

@@ -14,6 +14,7 @@
 #include <helper/binarybuffer.h>
 #include <helper/command.h>
 #include <rtt/rtt.h>
+#include <target/rtt.h>
 
 #include "target.h"