Fix empty compile_commands.json

This commit re-enables the keep_last_cmd member variable in the TaskBase class
(which was disabled by default in Waf 2.0.9) when clang_compilation_database.py
is used. The clang_compilation_database.py file was correctly updated in the
relevant Waf commit introducing the change [1], but is missing in ns-3's bundled
version of Waf.

[1] 7778868718

waf-tools/clang_compilation_database.py

@@ -15,9 +15,11 @@ Usage:
 """
 
 import sys, os, json, shlex, pipes
-from waflib import Logs, TaskGen
+from waflib import Logs, TaskGen, Task
 from waflib.Tools import c, cxx
 
+Task.TaskBase.keep_last_cmd = True
+
 if sys.hexversion >= 0x3030000:
 	quote = shlex.quote
 else: