adb: append the libwinpthread COPYING to adb's NOTICE.

We statically link libwinpthread into Windows binaries, but soong
currently can't specify notice files for toolchain_libraries. Add
libwinpthread's COPYING info directly to adb's as a hack.

Bug: http://b/36073965
Test: `make PRODUCT-sdk-win_sdk` + manual inspection of NOTICE.txt
Change-Id: I58d6be0ecf5626b67308f7e94128a2cd44dab161