Missing include when using DBoW2

[SteffenEhrle]

An include is missing

"util/converter.h" must be included to use function "util::converter::to_desc_vec" in "data/bow_vocabulary.cc".

To Reproduce

Using dbow2, compilation fails.

Steps to reproduce the behavior:
(Windows)

1. Use dbow2 instead of fbow
2. Apply this PR for DBoW2 binary loading and saving.

Expected behavior

Compilation error

Environment

• Hardware: [PC]
• CPU: [Intel]
• OS: [Windows 8]
• master

[SteffenEhrle]

#384 is the respective PR