module: fix linker error for MODULE_VERSION when !MODULE and CONFIG_SYSFS=n

lib/built-in.o:(__modver+0x8): undefined reference to `__modver_version_show'
lib/built-in.o:(__modver+0x2c): undefined reference to `__modver_version_show'

Simplest to just not emit anything: if they've disabled SYSFS they probably
want the smallest kernel possible.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/module.h

@@ -168,7 +168,7 @@ extern struct module __this_module;
   local headers in "srcversion".
 */
 
-#ifdef MODULE
+#if defined(MODULE) || !defined(CONFIG_SYSFS)
 #define MODULE_VERSION(_version) MODULE_INFO(version, _version)
 #else
 #define MODULE_VERSION(_version)					\