Portability fix: Build where sizeof is not compatible with int

On Debian amd64, for example:

 util.c: In function 'iaddr_to_string':
 util.c:575:3: error: field precision specifier '.*' expects argument
 of type 'int', but argument 3 has type 'long unsigned int'

Thanks to Jonathan Amery for the report.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>