monitor/hmp: : Include missing 'exec/target_long.h' header

The "exec/target_long.h" header is indirectly included, pulled
via "cpu.h" -> "exec/cpu-defs.h". Include it explicitly otherwise
we'd get when removing the latter:

  monitor/hmp-target.h:35:5: error: type name requires a specifier or qualifier
     35 |     target_long (*get_value)(Monitor *mon, const struct MonitorDef *md,
        |     ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20260427080738.77138-2-philmd@linaro.org>

include/monitor/hmp-target.h

@@ -28,7 +28,7 @@
 typedef struct MonitorDef MonitorDef;
 
 #ifdef COMPILING_PER_TARGET
-#include "cpu.h"
+#include "exec/target_long.h"
 struct MonitorDef {
     const char *name;
     int offset;