common-user: Only compile the common user code if have_user is set

There is no need to waste cycles here if we only compile the system
binaries or tools. Additionally, this change is even a hard requirement
for building the tools on systems that do not have an entry in the
common-user/host/ folder (since common-user/meson.build is trying
to add such a path via the include_directories() command).

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Message-Id: <20220622140328.383961-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

common-user/meson.build

+if not have_user
+   subdir_done()
+endif
+
 common_user_inc += include_directories('host/' / host_arch)
 
 user_ss.add(files(