Support building libwatchdog against musl

Support building libwatchdog against musl by moving the
sigev_notify_thread_id hack to only apply to glibc, musl provides
a #define.

Bug: 190084016
Test: m USE_HOST_MUSL=true libwatchdog
Change-Id: I47ce73014f0bcc0c40591192e4240381c441c67e
1 file changed