Remove *.cxx from .gitignore

[smessmer]

See noloader/cryptopp-cmake#66

I copied the cryptopp project into a subdirectory of my own project and add_subdirectory to crypto++ to use it as a dependency using cryptopp-cmake. Probably a somewhat common setup.
When trying to build my project, I got weird errors with missing test_....cxx files when trying to build my project. Turns out, this is because the TestPrograms/**.cxx files were missing from my repository because the crypto++ .gitignore (https://github.com/weidai11/cryptopp/blob/master/.gitignore#L6 ) prevented them from being added.

I assume other people will run into this issue as well and it's somewhat hard to figure out. Is it possible to remove *.cxx from the crypto++ .gitignore?

[noloader]

This should be cleared at Commit c15aeaa7b5cd.

I hope this does not come back to bite us in the ass.