API: C generator improve symbol protection.

Change-Id: I9ef042c06c12892a2bbc23cc3bac6891c77e3feb
Signed-off-by: Ole Troan <ot@cisco.com>
diff --git a/src/tools/vppapigen/vppapigen_c.py b/src/tools/vppapigen/vppapigen_c.py
index 2a66ff3..f0fb008 100644
--- a/src/tools/vppapigen/vppapigen_c.py
+++ b/src/tools/vppapigen/vppapigen_c.py
@@ -85,8 +85,8 @@
 
 
 def duplicate_wrapper_head(name):
-    s = "#ifndef defined_%s\n" % name
-    s += "#define defined_%s\n" % name
+    s = "#ifndef _vl_api_defined_%s\n" % name
+    s += "#define _vl_api_defined_%s\n" % name
     return s