mxml: remove SECURITY_CFLAGS override

mxml: remove SECURITY_CFLAGS override

mxml is not neeeded anymore as a dependency of agl-service-unicens
so the override on SECURITY_CLAGS can be removed

Bug-AGL: SPEC-1108

Change-Id: I704fdeed2db07a159a1cd32ebcf6ae93d8702a2b
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12171
Reviewed-by: Thomas Rini <trini@konsulko.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: José Bollo <jobol@nonadev.net>
Reviewed-by: Romain Forlot <romain.forlot@iot.bzh>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>