shared: move async.[ch] from src/basic/ → src/shared/

That way we can later add an async wrapper for rm_rf() which is in
src/shared/, too.

src/basic/meson.build

@@ -7,7 +7,6 @@ basic_sources = files(
         'architecture.c',
         'argv-util.c',
         'arphrd-util.c',
-        'async.c',
         'audit-util.c',
         'build.c',
         'bus-label.c',

src/shared/meson.build

@@ -4,6 +4,7 @@ shared_sources = files(
         'acpi-fpdt.c',
         'apparmor-util.c',
         'ask-password-api.c',
+        'async.c',
         'barrier.c',
         'base-filesystem.c',
         'battery-util.c',