devtools: fix build check for whether meson has run

devtools: fix build check for whether meson has run

The current check to see whether we need to call meson or just ninja
simply checked if the build directory existed. However, if meson was run
but failed, the build directory would still exist. We can fix this by
instead checking for the build.ninja file inside the directory. Once that
is present, we can use ninja safely and let it worry about rerunning
meson if necessary.

Fixes: a55277a788df ("devtools: add test script for meson builds")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>