Add qmake generated files to .gitignore.

Add source files generated by qmake for automatic static plugin
imports to .gitignore.

Change-Id: I2a2da2f8db41b41601566e66238affa3dcc66dff
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

.gitignore

@@ -312,3 +312,6 @@ tests/auto/dbus/qdbusinterface/qmyserver/qmyserver
 tests/auto/gui/kernel/qfileopenevent/qfileopeneventexternal/qfileopeneventexternal
 tests/auto/network/bearer/qnetworksession/lackey/lackey
 tests/auto/widgets/kernel/qapplication/modal/modal
+
+# Generated static plugin import sources
+*_plugin_import.cpp