make check timestamp when per-binary build

[YujiOshima]

When build per-binary, it doesn't check timestamp.
If there is a binary always make says below even the binary is old.

make build/infrakit-*
make: 'build/infrakit-*' is up to date.

This PR make it check *.go file time stamp.

Signed-off-by: YujiOshima yuji.oshima0x3fd@gmail.com

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a2e4ca7). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #553   +/-   ##
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  Coverage          ?   56.69%           
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  Files             ?       57           
  Lines             ?     3963           
  Branches          ?        0           
=========================================
  Hits              ?     2247           
  Misses            ?     1430           
  Partials          ?      286

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[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "fixmake" git@github.com:YujiOshima/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842356607768
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.