Add LOCAL_EXPORT_C_INCLUDES_DIRS to library builds.

It seems like a good idea to make use of the features provided by
the build system. Adding this means projects that depend on libz,
libz-host or libunz no longer need to include "external/zlib" in
their LOCAL_C_INCLUDES build variable.

Change-Id: Icec50daf387017ab26c44cc90beb1687b6d121b3
Merge "Add LOCAL_EXPORT_C_INCLUDES_DIRS to library builds."