Fix KFUNC_PROBE return value
The KFUNC_PROBE macro is using "void" as return type, this is causing problems
in some tools that have a filtering enable that returns 0.
Reproducer: (Notice that it requires BTF support)
```
$ python opensnoop.py --pid 5
/virtual/main.c:33:21: error: void function '____kretfunc__do_sys_open' should not return a value [-Wreturn-type]
if (pid != 5) { return 0; }
^ ~
1 error generated.
...
```
Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>
diff --git a/tools/opensnoop.py b/tools/opensnoop.py
index 995443e..28fe755 100755
--- a/tools/opensnoop.py
+++ b/tools/opensnoop.py
@@ -197,6 +197,8 @@
data.ret = ret;
events.perf_submit(ctx, &data, sizeof(data));
+
+ return 0:
}
"""