frameworks/base: Make sure Singleton instantiations are exported

This fixes using Singletons across shared library borders
with gcc 4.7+

Change-Id: I84b9cc99add4ace64c16e4fa8414a4fc7e4693c7
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

Conflicts:
	include/utils/Singleton.h