Fix a systrace warning when devel/subversion is installed.

--- build/version2code.py.orig	Sun Jul 31 13:02:11 2016
+++ build/version2code.py	Sat May  6 20:57:51 2017
@@ -6,7 +6,7 @@ from version import extractRevisionString, packageVers
 import sys
 
 def iterVersionInclude():
-	revision = extractRevisionString()
+	revision = "unknown"
 
 	yield '// Automatically generated by build process.'
 	yield 'const bool Version::RELEASE = %s;' % str(releaseFlag).lower()