stress-ng: disable DEBUG_BUILD

stress-ng: disable DEBUG_BUILD

We get do_compile error when DEBUG_BUILD is enabled:

  error: inlining failed in call to 'always_inline' '_mm512_storeu_si512': target specific option mismatch

To reproduce the issue, set the following lines in local.conf
and then `bitbake stress-ng'.

  MACHINE = "qemux86-64"
  DEBUG_BUILD = "1"

A new issue is filed for stress-ng to track it:
https://github.com/ColinIanKing/stress-ng/issues/315

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>