Do not pull Qt4 debug plugin DLLs

dlls_in_dir() grabs all files ending with *.dll, *.so and *.dylib in a certain
directory. On Windows this would grab debug copies of Qt 4 plugins, which then
causes PyInstaller to add a dependency on the Debug CRT __in addition__ to the
release CRT.
Merge pull request #100 from lvillani/windows-qt4-do-not-pull-debug-crt

Do not pull Qt4 debug plugin DLLs.