Fix packages/modules/DnsResolver/DnsProxyListener.h for incomplete type for unique_ptr

Fix invalid application of 'sizeof' to an incomplete type
'addrinfo' error.
See details in https://android-build.corp.google.com/artifact/pending/P75500812/aosp_cf_arm64_phone-userdebug/latest/view/logs%2Ferror.log

Bug: b/344681472
Test: Fetch clang-dev prebuilts from go/ab and change clang
configurations in global.go, then run mm to test.

Change-Id: I268047fad392ca2c657d7424345a672b9657bae5