Fix missing import.

Test: Verify that script runs properly now that import is present.
Change-Id: I255bc6ca2ba1f2cc387001c6f4bf9690a90eb47c

tools/releasetools/merge_target_files.py

@@ -80,6 +80,7 @@ import fnmatch
 import logging
 import os
 import shutil
+import subprocess
 import sys
 import zipfile