[vim] Adding a new line before the section of imports

... to satisfy pylint and fix the build.

```
Script:
--
/usr/bin/python /Users/buildnode/jenkins/workspace/oss-swift-incremental-RA-osx/swift/utils/python_lint.py
--
Exit Code: 1

Command Output (stdout):
--
./utils/vim/swift-format.py:36:1: I201 Missing newline before sections or imports.

--
```

Author: moiseev

utils/vim/swift-format.py

@@ -33,6 +33,7 @@
 import platform
 import subprocess
 import sys
+
 import vim
 
 binary = 'swift-format'