issue 2795: python 3 fix in Rust C header gen

The C header generation script was failing with a unicode error
in Python 3 on FreeBSD.  Fix the reading of files to properly
handle unicode in all Python 3 environments.

Redmine issue:
https://redmine.openinfosecfoundation.org/issues/2794

rust/gen-c-headers.py

@@ -169,7 +169,7 @@ def gen_headers(filename):
     if not should_regen(filename, output_filename):
         return
 
-    buf = open(filename).read()
+    buf = open(filename, "rb").read().decode("utf-8")
     writer = StringIO()
 
     for fn in re.findall(