meson: Move linux_user_ss to linux-user/

We have no need to reference linux_user_ss outside of linux-user.
Go ahead and merge it directly into specific_ss.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/meson.build

@@ -2,6 +2,8 @@ if not have_linux_user
    subdir_done()
 endif
 
+linux_user_ss = ss.source_set()
+
 common_user_inc += include_directories('host/' / host_arch)
 common_user_inc += include_directories('.')
 
@@ -42,3 +44,5 @@ subdir('sh4')
 subdir('sparc')
 subdir('x86_64')
 subdir('xtensa')
+
+specific_ss.add_all(when: 'CONFIG_LINUX_USER', if_true: linux_user_ss)