Add imp to windows freezer_includes

Got an ImportError on this line when I was testing on Windows.

https://github.com/terminalmage/salt/blob/43e15d8/salt/loader.py#L624

setup.py

@@ -781,6 +781,7 @@ class SaltDistribution(distutils.dist.Distribution):
 
         if IS_WINDOWS_PLATFORM:
             freezer_includes.extend([
+                'imp',
                 'win32api',
                 'win32file',
                 'win32con',