keymap-gen: Import sys

It is used in usage()

tools/keymap-gen

diff --git a/tools/keymap-gen b/tools/keymap-gen
index d79b2c8..e03c762 100755
--- a/tools/keymap-gen
+++ b/tools/keymap-gen
@@ -16,6 +16,7 @@
 import argparse
 import hashlib
 import time
+import sys
 
 class Database: