loader: Make device extensions return same count

loader: Make device extensions return same count

Previously the loader would return a different count in the call to
vkEnumeratePhysicalDeviceExtensionProperties if any implicit layers contained extensions
that a driver also happened to support, thus causing a duplicate entry to the count, but
was later resolved in the second call to that function. This commit de-duplicates the
extensions when getting the count, preventing such an issue from occuring.

Change-Id: I3b3c0b8871e412d26dda916ed1e1e3fe3299491e