misc: Add vkGetInstanceProcAddr() entrypoint

Adding this entrypoint as this has been discussed in khronos  and is needed
for the loader/layer/extension proposal.

Caveats:
1) Have not updated layers or any tests/demos yet that will come later;
2) No one including loader is using this call yet;
3) Leaving exisitng vkGetProcAddr() as is for now; later once loader
is using vkGetInstanceProcAddr call can switch vkGetProcAddr() to
vkGetDeviceProcAddr()
diff --git a/vulkan.py b/vulkan.py
index cb0b04f..e795da1 100755
--- a/vulkan.py
+++ b/vulkan.py
@@ -235,6 +235,10 @@
              Param("size_t*", "pDataSize"),
              Param("void*", "pData")]),
 
+        Proto("void*", "GetInstanceProcAddr",
+            [Param("VkInstance", "instance"),
+             Param("const char*", "pName")]),
+
         Proto("void*", "GetProcAddr",
             [Param("VkPhysicalDevice", "gpu"),
              Param("const char*", "pName")]),