Export init_mm, some modules need that

arch/ppc/kernel/process.c

@@ -56,6 +56,7 @@ static struct files_struct init_files = INIT_FILES;
 static struct signal_struct init_signals = INIT_SIGNALS(init_signals);
 static struct sighand_struct init_sighand = INIT_SIGHAND(init_sighand);
 struct mm_struct init_mm = INIT_MM(init_mm);
+EXPORT_SYMBOL(init_mm);
 
 /* this is 8kB-aligned so we can get to the thread_info struct
    at the base of it from the stack pointer with 1 integer instruction. */