docs: fix 'make doxygen' under python3

The 'make doxygen' component has this cool vpp specific customization called siphon.
This updates the siphon component so that 'make doxygen' works with python3.

Needed-By:  https://gerrit.fd.io/r/23159

Type: docs

Change-Id: Ie29f1602bf3460b637058acbb0a2f19b128a8824
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>
diff --git a/doxygen/siphon/process_syscfg.py b/doxygen/siphon/process_syscfg.py
index 94be591..bccde2c 100644
--- a/doxygen/siphon/process_syscfg.py
+++ b/doxygen/siphon/process_syscfg.py
@@ -14,7 +14,8 @@
 
 # Generate syscfg formatted output
 
-import process, parsers
+from . import process, parsers
+
 
 class SiphonSYSCFG(process.Siphon):