Update tutorial-ubuntu-16.04.md

Add the spell 
```
 -Wl,--export-dynamic
```

Author: ianamason

doc/tutorial-ubuntu-16.04.md

@@ -81,5 +81,5 @@ Turn the bitcode into a second executable binary. (optional -- just for fun and
 
 ```
 llc -filetype=obj httpd.bc
-gcc httpd.o -lpthread -lapr-1 -laprutil-1 -lpcre -o httpd.new
+clang httpd.o  -Wl,--export-dynamic -lpthread -lapr-1 -laprutil-1 -lpcre -o httpd_from_bc 
 ```