semihosting/syscalls: Include missing 'exec/cpu-defs.h' header

target_ulong is defined in each target "cpu-param.h",
itself included by "exec/cpu-defs.h".
Include the latter in order to avoid when refactoring:

  include/semihosting/syscalls.h:26:24: error: unknown type name 'target_ulong'
     26 |                        target_ulong fname, target_ulong fname_len,
        |                        ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250103171037.11265-2-philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250116160306.1709518-4-alex.bennee@linaro.org>

include/semihosting/syscalls.h

@@ -9,6 +9,7 @@
 #ifndef SEMIHOSTING_SYSCALLS_H
 #define SEMIHOSTING_SYSCALLS_H
 
+#include "exec/cpu-defs.h"
 #include "gdbstub/syscalls.h"
 
 /*