zsh: Explicitly set EXTRA_OEMAKE as required

* This recipe currently relies on EXTRA_OEMAKE having been to set to
  "-e MAKEFLAGS=" in bitbake.conf to operate. It is necessary to make
  this explicit so that the default in bitbake.conf can be changed.
* Unfortunately the build doesn't fail, it just produces almost empty
  output and QA warning is shown:
  WARNING: zsh-5.0.5-r0 do_package: QA Issue: zsh: Files/directories
  were installed but not shipped in any package:
    /usr
  Please set FILES such that these items are packaged. Alternatively if
  they are unneeded, avoid installing them or delete them within
  do_install.
  zsh: 1 installed and not shipped files. [installed-vs-shipped]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>