Move linker-namespace functions declarations

They are intended for platform use only and we shouldn't have them
in the public header file. This CL moves them to libnativeloader
private header file.

Bug: http://b/28174921
Change-Id: I01cf446d2d8d7cf6f9e31130aa3d4e54fbbbb9fc
(cherry picked from commit 6796522256b4cd861d3ebaf8264bd272b3c7441f)
diff --git a/libnativeloader/native_loader.cpp b/libnativeloader/native_loader.cpp
index 8103b37..488608a 100644
--- a/libnativeloader/native_loader.cpp
+++ b/libnativeloader/native_loader.cpp
@@ -19,7 +19,7 @@
 
 #include <dlfcn.h>
 #ifdef __ANDROID__
-#include <android/dlext.h>
+#include "dlext_namespaces.h"
 #include "cutils/properties.h"
 #define LOG_TAG "libnativeloader"
 #include "log/log.h"