util: Make qemu_oom_check() a static function

util: Make qemu_oom_check() a static function

The qemu_oom_check() function, which we define in both oslib-posix.c
and oslib-win32.c, is now used only locally in that file; make it
static.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220226180723.1706285-3-peter.maydell@linaro.org