fix undeclared executables

[mrustl]

checking for executable files ... WARNING
Found the following executable files:
inst/extdata/engine/linux/vs2dh3_3.exe
inst/extdata/engine/win/vs2dh3_3.exe
inst/extdata/engine/win/vs2dt3_3.exe
Source packages should not contain undeclared executable files.
See section ‘Package structure’ in the ‘Writing R Extensions’ manual.

See: https://api.travis-ci.org/v3/job/392562091/log.txt

As an first easy solution they should be explicitly listed