math/qtoctave: Unbreak build with Clang 6

qtoctave/src/generate_menu.cpp:282:64: error: cannot initialize return object of type 'MenuExtCallBack *' with an rvalue of type 'bool'

if (!file.open(QIODevice::ReadOnly | QIODevice::Text)) return false;
                                                              ^~~~~

http://beefy3.nyi.freebsd.org/data/112amd64-quarterly/482201/logs/qtoctave-0.10.1_4.log

PR: 232985
Submitted by: w.schwarzenfeld@utanet.at