Linguist examples pollute the source tree with *_en.ts files

When I do a top-level out-of-source build of Qt with -make examples in effect, I consistently get git grumbles about qttools/examples/linguist/{arrowpad/arrowpad,hellotr/hellotr,trollprint/trollprint}_en.ts lying around in the checked out source tree next time I'm doing an all-module update.
Note that this happens even though I have not run any qttools examples; this is generated when building the examples.

• Out-of-source builds should not normally generate anything in the source directories.
• Where builds do generate files in any source directory, these files should be overtly mentioned in relevant .gitignore files.