Mark base64.h encode and decode API's as inline

[f2013519]

The b64strlen & b64decode API's are defined in a header and will cause linker issues if included in multiple files. We ran into this issue while attempting to use this file in our project.

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[f2013519]

cc @masahi Could you pls. take a look, thanks.

[junrushao]

That makes sense. Thanks for the contribution!

[masahi]

@f2013519 Good catch!

[f2013519]

Thanks!