adding entry point to soft driver, adding C test

src/soft/lib.zig

@@ -1,7 +0,0 @@
-const std = @import("std");
-
-const common = @import("common");
-
-test {
-    std.testing.refAllDeclsRecursive(@This());
-}

src/soft/libvulkan.zig

@@ -0,0 +1,11 @@
+const std = @import("std");
+const vk = @import("vulkan");
+const common = @import("common");
+
+pub export fn vk_icdGetInstanceProcAddr(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 common.icd.getInstanceProcAddr(instance, name);
+}