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