icd: Move include to header to fix clang build
icd: Move include to header to fix clang build

Move include <string> to header file to fix
clang build when using libstdc++.

Fixes #2303

Change-Id: Id2fc97be7cb83fa12ee1495ef57177c0a7dad893

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers