ExternalProject: Avoid infinite loop on file download hash mismatch

When the loop was added by commit v3.0.0-rc1~103^2~3 (ExternalProject:
Reattempt download when verification fails, 2015-01-15), we forgot to
actually increment the loop counter.  Add the increment line.
Merge topic 'ExternalProject-download-loop' into next

9f49ac3d ExternalProject: Avoid infinite loop on file download hash mismatch