Fix the build for Android Nougat.

The dladdr(3) manpage says that <dlfcn.h> is required. On Linux, the
build succeeded without it because build_id.c includes <link.h> which
includes <dlfcn.h>. On Android, we must include <dlfcn.h> directly.

Fixes: 5c98d382

 "util: Query build-id by symbol address, not library name"
Reviewed-by: Matt Turner <mattst88@gmail.com>