lib: vsprintf: fix API build

lib: vsprintf: fix API build

Avoid a build failure when building with CONFIG_API=y, CONFIG_EXAMPLES=y:

    lib/vsprintf.c:312:14: warning:
    ‘device_path_string’ defined but not used [-Wunused-function]
     312 | static char *device_path_string(char *buf, char *end, void *dp, int field_width,
         |              ^~~~~~~~~~~~~~~~~~

Fixes: 64b5ba4d293a ("efi_loader: make device path to text protocol customizable")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>