meson: Share common C source prefixes

meson: Share common C source prefixes

gnu_source_prefix defines _GNU_SOURCE for compiler object functions.
The definition is universally available in the code base.

docs/devel/style.rst also says that the "qemu/osdep.h" header is
always included, so files included in the file is also universally
available in the code base.

Rename gnu_source_prefix to osdep_prefix, and add #include directives
that are referred by the users of gnu_source_prefix and contained in
qemu/osdep.h to safely de-duplicate #include directives.

Cc: qemu-stable@nongnu.org
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250424-buildsys-v1-3-97655e3b25d7@daynix.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>