extras/dialer: move dialer.go to sub directory

`extras/dialer` contains both a dialer script and dialer.go, the go
sources for the dialer server and client, to allow users to build the
dialer container themselves. Move dialer.go to a sub directory `src`
since `make clean` (running `go clean ./...`) deletes the dialer script
otherwise.

Fixup for 4d20a91a97

Signed-off-by: Michael Schubert <michael@kinvolk.io>

extras/dialer/README.md

@@ -41,6 +41,6 @@ sudo ./tools/dialer/time-scope-probe 3 3
 ## Build dialer container
 
 ```
-go build -o bin/dialer
+go build -o bin/dialer src/dialer.go
 docker build -t dialer .
 ```