fix(src): reinit vsag by explicitly calling init()

[Coien-rr]

related: #108

if we use VSAG by static lib, we need explicitly call vsag::init()
but it will reinit VSAG, since init() has been called to initializing _init. https://github.com/alipay/vsag/blob/185eccffd8b3db6e77c3c24d378c40bc15243d36/src/vsag.cpp#L61
although reinitializing VSAG does not cause bugs yet.
But this code is not elegant, on the other hand, the init log will repeat twice in debug mode

[jiaweizone]

LGTM