kmod-setup: don't warn when ipv6 can't be loaded

Not having IPv6 is a valid setup. Let's not print a warning in that
case.

Addresses:

  https://bugs.freedesktop.org/show_bug.cgi?id=87475

(cherry picked from commit b4aa82f168)

src/core/kmod-setup.c

@@ -66,7 +66,7 @@ int kmod_setup(void) {
                 { "autofs4",   "/sys/class/misc/autofs",    true,  NULL                },
 
                 /* early configure of ::1 on the loopback device */
-                { "ipv6",      "/sys/module/ipv6",          true,  NULL                },
+                { "ipv6",      "/sys/module/ipv6",          false,  NULL               },
 
                 /* this should never be a module */
                 { "unix",      "/proc/net/unix",            true,  NULL                },