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>
diff --git a/exec.c b/exec.c
index 7728ea3..7e22980 100644
--- a/exec.c
+++ b/exec.c
@@ -66,8 +66,8 @@
 AddressSpace address_space_memory;
 DMAContext dma_context_memory;
 
-MemoryRegion io_mem_rom, io_mem_unassigned, io_mem_notdirty;
-static MemoryRegion io_mem_subpage_ram;
+MemoryRegion io_mem_rom, io_mem_notdirty;
+static MemoryRegion io_mem_unassigned, io_mem_subpage_ram;
 
 #endif