kernel/fork.c: export kernel_thread() to modules

mutex-subsystem-synchro-test-module.patch needs this

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/fork.c

@@ -2683,6 +2683,7 @@ pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 
 	return kernel_clone(&args);
 }
+EXPORT_SYMBOL(kernel_thread);
 
 #ifdef __ARCH_WANT_SYS_FORK
 SYSCALL_DEFINE0(fork)