isaspec : fix isaspec build error in aosp
in Android 12 build have error "ninja:
'external/mesa/src/compiler/isaspec/README.rst', needed by
'out/target/product/s/obj/MESON_MESA3D_GEN/.timestamp', missing and
no known rule to make it" because commit:
d48d8aefdff44e8a6ece030a782dc9d152bb1d5d (docs: Move isaspec out of
drivers/freedreno) modify isaspec.rst Location.
Signed-off-by: cheyang <cheyang@bytedance.com>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25697>