Define `SO_BINDTOIFINDEX` on Android

Android supports SO_BINDTOIFINDEX since [SDK level 31](https://cs.android.com/android/platform/superproject/main/+/main:prebuilts/vndk/v31/arm/include/bionic/libc/kernel/uapi/asm-generic/socket.h;l=88;drc=684b16d3ce1e891ebe15d5678e12fa05ee6dd6e4), which is [introduced to support kernel v5.1.3](https://android-review.git.corp.google.com/c/platform/bionic/+/965472).