.gitignore: ignore .exe files (for Windows)

.gitignore: ignore .exe files (for Windows)

Make git ignore .exe files, which are generated by Windows
cygwin builds.

Examples:
        new file:   fio.exe
        new file:   t/axmap.exe
        new file:   t/fio-dedupe.exe
        new file:   t/fio-genzipf.exe
        new file:   t/fio-verify-state.exe
        new file:   t/gen-rand.exe
        new file:   t/ieee754.exe
        new file:   t/lfsr-test.exe
        new file:   t/stest.exe

Signed-off-by: Jens Axboe <axboe@kernel.dk>