Ensure that a link to pc-bios/qemu_vga.ndrv is added to the build tree,
otherwise the optional MacOS client driver will not be loaded by OpenBIOS
when launching QEMU directly from the build directory.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210831165020.84855-1-programmingkidx@gmail.com>
[lv: commit message rewording as suggested by Mark]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>