Fix (-W)documentation error found by Clang

Change-Id: I2d2bbc211e98ad24a27c855031f670b7bdbd530e
Reviewed-on: https://code.wireshark.org/review/3162
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>