dlext_namespace.h: Declare android_get_LD_LIBRARY_PATH

This allows clients creating their own namespace do so by modifying
the default library path, rather than hardcoding it multiple places in
the system.

Bug: 33531483
Test: build
Change-Id: I321e219392b38c235b220986e1520b49a4669863
(cherry picked from commit 823c266a)