prl files of installed libraries mess up build due to containing -L entries for system paths

In qmake's C++ code, UnixMakefileGenerator::findLibraries()(in generators/unix/unixmake.cpp), a variable libidx is initialized as 0, which is used as the number of items in QList<QMakeLocalFileName> libdirs without being set to the right value, which results in some unexpected problem when the function is going to remove something like -L/usr/lib64. And my patch can fix my problem.