Update Cbc_C_Interface.cpp

[DrStriky]

It happens that parts of the solution log are stuck in the buffer. This should flush the buffer before the solve function returns.

[CLAassistant]

All committers have signed the CLA.

[svigerske]

I don't know why there is an Attic subdirectory in a code repository that has version-control, but it will probably not help anything to add some flush() there.

Also, the message handler may not have been written to stdout/stderr.

[tkralphs]

The Attic subdirectory was put there to make it easy to restore the files that were massively changed to what they were before merging #374 (big refactoring), since @johnjforrest wanted to be able to switch back and forth easily without doing git gymnastics and also wanted others to be able to do it if needed. When we first merged the refactor branch, there were initially some hiccups but now everything seems to be smoothed out. At some point, we can delete the Attic subdirectory, especially since it is easy to go back and get those files if really needed.