Support building diagnose_usb against musl

Support building diagnose_usb against musl by reimplementing
group_member, which doesn't exist in musl and isn't used anywhere
else in the tree.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: Id0fe51d0bd8677561d0bbdb72d3b1a956127a11c