Updated the submodule subsurface

The earlier submodule was mapped to commit
e0610361518e71c6d4964d3c3cbd0a6097399034 of
https://github.com/glance-/subusurface
which is non-existent now. This was the reason for the error:

fatal: reference is not a tree:
e0610361518e71c6d4964d3c3cbd0a6097399034
Unable to checkout 'e0610361518e71c6d4964d3c3cbd0a6097399034' in
submodule path 'subsurface'

I have changed it to the latest commit
7bc878c0b957debc7c50c6db05ecdec73f24d799

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>