sysemu/xen: Add missing 'exec/cpu-common.h' header for ram_addr_t type

As this header use the ram_addr_t type, it has to include
"exec/cpu-common.h" to avoid odd errors such:

  include/sysemu/xen.h:35:44: error: unknown type name 'ram_addr_t'; did you mean 'in_addr_t'?
   35 | static inline void xen_hvm_modified_memory(ram_addr_t start, ram_addr_t length)
      |                                            ^~~~~~~~~~
      |                                            in_addr_t

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200908155530.249806-4-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/sysemu/xen.h

@@ -8,6 +8,8 @@
 #ifndef SYSEMU_XEN_H
 #define SYSEMU_XEN_H
 
+#include "exec/cpu-common.h"
+
 #ifdef NEED_CPU_H
 # ifdef CONFIG_XEN
 #  define CONFIG_XEN_IS_POSSIBLE