Improve the ObjC Generated Interface Header Name definition

[giomfo]

Currently, the Element project defined this header with $(SWIFT_MODULE_NAME)-Swift.h

The problem is the resulting header name depends on the selected target.

This complicates the import of this header in the source code shared between an extension and the application itself

#ifdef IS_SHARE_EXTENSION
#import "RiotShareExtension-Swift.h"
#else
#import "Riot-Swift.h"
#endif

I suggest here defining a unique header name for all the targets

then the import will be everywhere:

#import "GeneratedInterface-Swift.h"