meson: cleanup common-user/ build

meson: cleanup common-user/ build

It is not necessary to have a separate static_library just for common_user
files; using the one that already covers the rest of common_ss is enough
unless you need to reuse some source files between emulators and tests.
Just place common files for all user-mode emulators in common_ss,
similar to what is already done for softmmu_ss in full system emulators.

The only disadvantage is that the include_directories under bsd-user/include/
and linux-user/include/ are now enabled for all targets rather than only
user mode emulators.  This however is not different from how include/sysemu/
is available when building user mode emulators.

Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>