igb_uio: pass MODULE_CFLAGS in Kbuild

igb_uio: pass MODULE_CFLAGS in Kbuild

With the legacy build system MODULE_CFLAGS can be set to pass compiler
flags specific for the kernel modules builds.
This is used currently by Ubuntu and Debian.
Set ccflags-y in the Kbuild to achieve the same result with Meson, and
to keep backward compatbility with older scripts.

Fixes regression in Ubuntu/Debian when the Kbuild is included in the
DKMS source package, as DKMS will pick it up silently by default if
present, causing the MODULE_CFLAGS to be ignored.

Fixes: a52f4574f798 ("igb_uio: build with meson")
Cc: stable@dpdk.org
Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>