hello: include hello.h

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

Detected through 'sparse' tool.

Change-Id: I6197f21c857833dafc3d6e3b750c764bf4610abd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7671
Tested-by: jenkins

src/hello.c

@@ -8,6 +8,7 @@
 #include <config.h>
 #endif
 #include <helper/log.h>
+#include "hello.h"
 
 COMMAND_HANDLER(handle_foo_command)
 {