meson: Do not include headers in source lists

Meson+ninja+compiler do this for us and are better at it.

https://mesonbuild.com/FAQ.html#do-i-need-to-add-my-headers-to-the-sources-list-like-in-autotools

src/nspawn/meson.build

@@ -2,35 +2,19 @@
 
 libnspawn_core_sources = files(
         'nspawn-bind-user.c',
-        'nspawn-bind-user.h',
         'nspawn-cgroup.c',
-        'nspawn-cgroup.h',
         'nspawn-creds.c',
-        'nspawn-creds.h',
-        'nspawn-def.h',
         'nspawn-expose-ports.c',
-        'nspawn-expose-ports.h',
         'nspawn-mount.c',
-        'nspawn-mount.h',
         'nspawn-network.c',
-        'nspawn-network.h',
         'nspawn-oci.c',
-        'nspawn-oci.h',
         'nspawn-patch-uid.c',
-        'nspawn-patch-uid.h',
         'nspawn-register.c',
-        'nspawn-register.h',
         'nspawn-seccomp.c',
-        'nspawn-seccomp.h',
         'nspawn-settings.c',
-        'nspawn-settings.h',
         'nspawn-setuid.c',
-        'nspawn-setuid.h',
         'nspawn-stub-pid1.c',
-        'nspawn-stub-pid1.h',
         'nspawn-util.c',
-        'nspawn-util.h',
-        'nspawn.h',
 )
 
 nspawn_gperf_c = custom_target(