Make sure std::size_t is available before it is used

Make sure std::size_t is defined before using it in the is_array
template.
Its absence causes an error when building with clang.

Change-Id: If999af1bd8a83905fb4c5b6d67d2d811cfc50a73
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>