Use .sinclude for bsd.sanitizer.mk

We don't install this file since MK_ASAN/MK_UBSAN is only supported for
src builds. However, some ports also use bsd.lib.mk/bsd.prog.mk so we
should not fail the build if it can't be included.

Reported by: jkim
Fixes: 7bc797e3f380 ("Add build system support for ASAN+UBSAN instrumentation")