Fix Swift Package Manager support

[woxtu]

Hi,

The current release installed via SPM cannot import headers as follows:

#import <FMDB/FMDB.h>

This PR creates the public headers directory to fix that.

[ccgus]

Why not import them via #import "FMDB.h", or just what is needed?

[Jeehut]

I import via @import FMDB;, did you try that.

[GLinnik21]

Why not import them via #import "FMDB.h", or just what is needed?

I think because the #import syntax is inconsistent between SPM and CocoaPods.