classes-global/staging: Exclude do_create_spdx from automatic sysroot extension

classes-global/staging: Exclude do_create_spdx from automatic sysroot extension

do_create_spdx is a outlier in that it doesn't need the RSS to be
extended just because it depends on do_populate_sysroot. In fact, it
only depends on do_populate_sysroot so it can see the actual recipes
sysroot, and attempting to extend the sysroot can cause problems for
some recipes (e.g. if a recipe does do_populate_sysroot[noexec] = "1")

As such, explicitly exclude do_create_spdx from extending the sysroot
just because it depends on do_populate_sysroot.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>