exec: make io_mem_unassigned private

There is no reason to avoid a recompile before accessing unassigned
memory.  In the end it will be treated as MMIO anyway.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/exec/cpu-common.h

@@ -111,7 +111,6 @@ void cpu_physical_memory_write_rom(hwaddr addr,
                                    const uint8_t *buf, int len);
 
 extern struct MemoryRegion io_mem_rom;
-extern struct MemoryRegion io_mem_unassigned;
 extern struct MemoryRegion io_mem_notdirty;
 
 #endif