linux-user: Don't include gdbstub.h in qemu.h

linux-user: Don't include gdbstub.h in qemu.h

Currently the linux-user qemu.h pulls in gdbstub.h. There's no real reason
why it should do this; include it directly from the C files which require
it, and drop the include line in qemu.h.

(Note that several of the C files previously relying on this indirect
include were going out of their way to only include gdbstub.h conditionally
on not CONFIG_USER_ONLY!)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210908154405.15417-9-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>