Make the <android/dlext.h> docs show up on the web.

Just enough doxygen so that
https://developer.android.com/ndk/reference/dlext_8h.html won't be completely
devoid of all this.

Bug: N/A
Test: ran doxygen manually
Change-Id: I1e0667feae93227e32c1bd671065359f5be85d9d