fsdev/9p-marshal: move G_GNUC_PRINTF to header

fsdev/9p-marshal: move G_GNUC_PRINTF to header

v9fs_string_sprintf() is annotated with G_GNUC_PRINTF(2, 3) in
9p-marshal.c, but the prototype in fsdev/9p-marshal.h is missing the
attribute, so callers that include only the header do not get format
checking.

Move the annotation to the header and delete the duplicate in the
source file. No behavior change.

Signed-off-by: Sean Wei <me@sean.taipei>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250613.qemu.9p.01@sean.taipei>
[CS: fix code style (max. 80 chars per line)]
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>