[dovecot/core] f82cd1: Makefile: Fix make distcheck when using scan-build

[GitHub]

  Branch: refs/heads/master-2.2
  Home:   https://github.com/dovecot/core
  Commit: f82cd18fa35c4634978de742c0706ce6c92f2a11
      https://github.com/dovecot/core/commit/f82cd18fa35c4634978de742c0706ce6c92f2a11
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-04-11 (Tue, 11 Apr 2017)

  Changed paths:
    M Makefile.am

  Log Message:
  -----------
  Makefile: Fix make distcheck when using scan-build

The _build directory shouldn't be deleted afterwards or it'll just fail.