[dovecot/core] 38bf95: Makefile: Fix make distcheck when using scan-build

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: 38bf959348d9d25477807476b34c417279911dbb
      https://github.com/dovecot/core/commit/38bf959348d9d25477807476b34c417279911dbb
  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.