pps: use cflags-y instead of EXTRA_CFLAGS

pps: use cflags-y instead of EXTRA_CFLAGS

Commit f77bf01425b1 ("kbuild: introduce ccflags-y, asflags-y and
ldflags-y") deprecates use of EXTRA_CFLAGS in the kernel build.

This has been cleaned up in the whole kernel tree long ago, but this one
single place must have been missed.

Replace the EXTRA_CFLAGS use by the common pattern for such debug flags.
No functional change.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Link: https://lore.kernel.org/r/20240306120515.15711-1-lukas.bulwahn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>