Fix dll installation on Windows

[myd7349]

According the CMake docs(https://cmake.org/cmake/help/latest/command/install.html):

Executables are treated as RUNTIME objects, except those marked with the MACOSX_BUNDLE property on macOS (see BUNDLE below.) For DLL platforms (all Windows-based systems including Cygwin), the DLL part of a shared library is treated as a RUNTIME target.

Here, I use bin instead of CMAKE_INSTALL_BINDIR, which is defined in GNUInstallDirs.cmake.

[lotem]

Thanks for the pull request.
Merged.