Fix missing public header in frameworks

[dymv]

Since update from 4.14.2 to 4.15.8 we noticed the following issue when compiling our project with PubNub framework built via Carthage:

<module-includes>:1:9: note: in file included from <module-includes>:1:
#import "Headers/PubNub.h"
        ^
.../Carthage/Build/iOS/PubNub.framework/Headers/PubNub.h:63:9: note: in file included from .../Carthage/Build/iOS/PubNub.framework/Headers/PubNub.h:63:
#import "PubNub+Publish.h"
        ^
.../Carthage/Build/iOS/PubNub.framework/Headers/PubNub+Publish.h:3:9: note: in file included from .../Carthage/Build/iOS/PubNub.framework/Headers/PubNub+Publish.h:3:
#import "PNPublishFileMessageRequest.h"
        ^
.../Carthage/Build/iOS/PubNub.framework/Headers/PNPublishFileMessageRequest.h:1:9: error: 'PNBasePublishRequest.h' file not found
#import "PNBasePublishRequest.h"
        ^
.../PubNubStreamHandler.swift:4:8: error: could not build Objective-C module 'PubNub'
import PubNub
       ^

To fix this, the PR moves PNBasePublishRequest.h to the Public Group. Note: the change was made for each platform iOS, tvOS, watchOS, OSX (according to framework names), but only iOS was tested locally.

[parfeon]

@dymv thanks for reporting this issue.

It is surprising, that test builds of CocoaPods (1.9.3) and Carthage (0.36.0) on Travis didn't detected this misconfiguration.
We will make changes and notify about release.

[client-engineering-bot]

@dymv your suggested changes are addressed in v4.15.9