.gitignore: move *.exe pattern to the top gitignore for Cygwin

GCC on Cygwin generates executables with .exe extension,
for example:
 scripts/basic/fixdep.exe
 scripts/docproc.exe

To ignore them, *.exe pattern should be moved
from tools/.gitignore to ./.gitignore

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>