Restrict private headers visibility.

[VasilErema]

If we use use_modular_headers! in Podfile we have all headers in umbrella header. This change allow to leave only one public header BLWebSocketsServer.h

[VasilErema]

BLWebSocketsServer-umbrella.h is now looks like this:

#ifdef __OBJC__
#import <UIKit/UIKit.h>
#else
#ifndef FOUNDATION_EXPORT
#if defined(__cplusplus)
#define FOUNDATION_EXPORT extern "C"
#else
#define FOUNDATION_EXPORT extern
#endif
#endif
#endif

#import "BLWebSocketsServer.h"

FOUNDATION_EXPORT double BLWebSocketsServerVersionNumber;
FOUNDATION_EXPORT const unsigned char BLWebSocketsServerVersionString[];