fixing module export

src/vulkan/lib.zig

@@ -1,10 +1,18 @@
 const std = @import("std");
+const vk = @import("vulkan");
 
 pub const icd = @import("icd.zig");
-
 pub const Instance = @import("Instance.zig");
 pub const PhysicalDevice = @import("PhysicalDevice.zig");
 
+pub export fn vkGetInstanceProcAddr(instance: vk.Instance, pName: ?[*:0]const u8) callconv(vk.vulkan_call_conv) vk.PfnVoidFunction {
+    if (pName == null) {
+        return null;
+    }
+    const name = std.mem.span(pName.?);
+    return icd.getInstanceProcAddr(instance, name);
+}
+
 test {
     std.testing.refAllDeclsRecursive(@This());
 }