Update haskeline submodule to 0.7.2.1 release tag

This fixes compilation under windows with pre-AMP GHCs
although it's not clear if bootstrapping GHC was affected.
(see https://github.com/judah/haskeline/issues/20 for details)

(cherry picked from commit 899cb3e7)

haskeline @ 7a72748b

-Subproject commit dbffdd9fc601fc0b74e9e43d237dba0a10b1e0fe
+Subproject commit 7a72748b3898e68c99e3d5ae7d06c58dd190b149