hw/riscv/opentitan: Include missing 'exec/address-spaces.h' header

opentitan_machine_init() calls get_system_memory(),
which is declared in "exec/address-spaces.h". Include
it in order to avoid when refactoring unrelated headers:

  hw/riscv/opentitan.c:83:29: error: call to undeclared function 'get_system_memory'
     83 |     MemoryRegion *sys_mem = get_system_memory();
        |                             ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20250206181827.41557-4-philmd@linaro.org>

hw/riscv/opentitan.c

@@ -28,6 +28,7 @@
 #include "hw/riscv/boot.h"
 #include "qemu/units.h"
 #include "system/system.h"
+#include "exec/address-spaces.h"
 
 /*
  * This version of the OpenTitan machine currently supports