layers: Fix linux compilation errors of isnan
layers: Fix linux compilation errors of isnan

Added std:: to isnan invocation.

Change-Id: I1353c4116497a88625a12e4003121716ec66a4f0

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools