loader: Add dynamic dispatch for unknown device extension entrypoints

GetInstancePorcAddr() is specified to return trampoline entrypoints for all
Vulkan core and extension entrypoints that are dispatched on an instance object
or a child of that instance object. However, typically, device extensions would
be unknown to the loader (don't want to rev the loader everytime an IHV creates
a new device extension).

This patch allows loader to dynamically discover device extension entrypoints
and configure generic trampoline code for these discovered device extensions.

include/vulkan/vk_layer.h

@@ -17,8 +17,7 @@
 #  define VK_LAYER_EXPORT
 #endif
 
-typedef void * (*PFN_vkGPA)(void* obj, const char * pName);
-
+typedef void * (VKAPI_PTR *PFN_vkGPA)(void* obj, const char * pName);
 typedef struct VkBaseLayerObject_
 {
     PFN_vkGPA pGPA;