Ben Gamari authored Jul 31, 2017 and Ben Gamari committed Aug 01, 2017

gcc complains about this while building with Hadrian,
```
rts/RetainerProfile.c: In function ‘computeRetainerSet’:

rts/RetainerProfile.c:1758:18: error:
     error: variable ‘rtl’ set but not used
[-Werror=unused-but-set-variable]
         RetainerSet *rtl;
                      ^~~
     |
1758 |     RetainerSet *rtl;
     |                  ^
```

Reviewers: austin, erikd, simonmar, Phyx

Reviewed By: Phyx

Subscribers: Phyx, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3801