gdbstub: move enums into separate header

This is an experiment to further reduce the amount we throw into the
exec headers. It might not be as useful as I initially thought because
just under half of the users also need gdbserver_start().

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240620152220.2192768-3-alex.bennee@linaro.org>

gdbstub/user.c

@@ -18,6 +18,7 @@
 #include "exec/gdbstub.h"
 #include "gdbstub/syscalls.h"
 #include "gdbstub/user.h"
+#include "gdbstub/enums.h"
 #include "hw/core/cpu.h"
 #include "trace.h"
 #include "internals.h"