adding header pragma for printf ffi example

I think the manual should not have code that does not compile. The ffi example in the manual for importing printf is causes problems when an echo precedes it.

Example (this code does not compile on 0.9.4)
```
echo("something")
proc printf(formatstr: cstring) {.importc: "printf", varargs.} #this line is from the maunal
printf("hello")
```

When adding the header pragma it works.