Makefile: don't add -Wno-maybe-uninitialized twice

Avoid adding the same flag to KBUILD_CFLAGS twice.

Fixes: 8602d97c ("Makefile: avoid false positive -Wmaybe-uninitialized")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>