Export include dir
Export include dir

Every user of this library adds external/lz4/lib to their
LOCAL_C_INCLUDES. Export it for them so that they don't need to hardcode
this path.

Change-Id: I66cfb947f78c3c13d001a21caa875263c0377d8f

omnirom/android_external_lz4,omnirom/android_external_lz4,omnirom/android_external_lz4