meson: automatically add 'extract' to 'sources'

In all cases, everything that we list in 'extract', we also list in
'sources'. We can simplify things by automatically appending the first
list to the second.

In the listings, move 'extract' key right below 'sources', since now
they are both "sources", just with slightly different meanings.

src/integritysetup/meson.build

@@ -7,10 +7,7 @@ endif
 executables += [
         libexec_template + {
                 'name' : 'systemd-integritysetup',
-                'sources' : files(
-                        'integrity-util.c',
-                        'integritysetup.c',
-                ),
+                'sources' : files('integritysetup.c'),
                 'extract' : files('integrity-util.c'),
                 'dependencies' : libcryptsetup,
         },