VPPAPIGEN: use SOURCE_DATE_EPOCH for datestring

bwiedemann@suse.com pointed me that:
For reproducible builds datestring should be generated using
SOURCE_DATE_EPOCH which distributions set centrally, and have build
tools that consume it to produce reproducible output.

https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal#Python

Change-Id: Iefa2b93231bbc4c8af49736be4d7a5720c46b28d
Signed-off-by: Nirmoy Das <nirmoy.aiemd@gmail.com>

src/tools/vppapigen/C.py

@@ -1,8 +1,9 @@
 # C generation
 import datetime
 import os
+import time
 
-datestring = datetime.datetime.now()
+datestring = datetime.datetime.utcfromtimestamp(int(os.environ.get('SOURCE_DATE_EPOCH', time.time())))
 input_filename = 'inputfil'
 top_boilerplate = '''\
 /*