Fix crash on case sensitive filesystems on Macos

Mixxx failed to launch on case-sensitive filesystems on MacOS
because it was unable to find its Qt plugins: it looks for
`PlugIns` rather than `plugins`, which is what the directory was
called.

CMakeLists.txt

@@ -2664,7 +2664,7 @@ if(APPLE AND MACOS_BUNDLE)
             get_filename_component(_qt_plugin_file "${_qt_plugin_path}" NAME)
             get_filename_component(_qt_plugin_type "${_qt_plugin_path}" PATH)
             get_filename_component(_qt_plugin_type "${_qt_plugin_type}" NAME)
-            set(_qt_plugin_dest "${_prefix}/Contents/plugins/${_qt_plugin_type}")
+            set(_qt_plugin_dest "${_prefix}/Contents/PlugIns/${_qt_plugin_type}")
             install(FILES "${_qt_plugin_path}"
                 DESTINATION "${_qt_plugin_dest}")
             set(${_qt_plugins_var}