kvargs: fix building with meson outside source tree

kvargs: fix building with meson outside source tree

When building DPDK with meson with the <builddir> being outside the source
directory, an error was generated as the path to the EAL headers was not
found. The path specified for the includes backed out unnecessarily far and
so broke when the build directory was not in the expected location of
just inside the source dir. Changing the path to be shorter, just going
down one level and then into EAL builds for both cases - where builddir is
in source directory, and also when it's outside it.

Fixes: 092ee516496d ("kvargs: build before EAL")

Reported-by: Anatoly Burakov <anatoly.burakov@intel.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Anatoly Burakov <anatoly.burakov@intel.com>