Makefile: followup on #2585

Makefile: followup on #2585

Fix a typo in `tag` target, double quote are missing here.

Without them, the `make tag` command fails like this:

```
if [[ "v3.0.35" ==  ]]; then \
            echo "e5f2df8 on stable-3.0 is already tagged as v3.0.35"; \
            exit 1; \
        fi
/bin/sh: -c: line 0: unexpected argument `]]' to conditional binary operator
/bin/sh: -c: line 0: syntax error near `;'
/bin/sh: -c: line 0: `if [[ "v3.0.35" ==  ]]; then     echo "e5f2df8 on stable-3.0 is already tagged as v3.0.35";     exit 1; fi'
make: *** [tag] Error 2
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>