.gitignore: Do not track `defconfig` from `make savedefconfig`

Running `make savedefconfig` creates by default `defconfig`, which is,
currently, on git’s radar, for example, `git status` lists this file as
untracked.

So, add the file to `.gitignore`, so it’s ignored by git.
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

.gitignore

@@ -143,6 +143,9 @@ x509.genkey
 /allrandom.config
 /allyes.config
 
+# Kconfig savedefconfig output
+/defconfig
+
 # Kdevelop4
 *.kdev4