Swift project support

Merge patch required to support Swift projects, from Avery Pierce.

Patch taken from https://github.com/matrix-org/matrix-ios-sdk/files/712987/olm_patch.txt.

Sign-off at https://github.com/matrix-org/matrix-ios-sdk/pull/220#issue-201470911.

OLMKit.podspec

@@ -32,6 +32,7 @@ Pod::Spec.new do |s|
   }
 
   s.source_files = "xcode/OLMKit/*.{h,m}", "include/**/*.{h,hh}", "src/*.{c,cpp}", "lib/crypto-algorithms/sha256.c",  "lib/crypto-algorithms/aes.c", "lib/curve25519-donna/curve25519-donna.c"
+  s.private_header_files = "xcode/OLMKit/*_Private.h"
 
   # Those files (including .c) are included by ed25519.c. We do not want to compile them twice
   s.preserve_paths = "lib/ed25519/**/*.{h,c}"

xcode/OLMKit/OLMKit.h

@@ -18,10 +18,6 @@
 
 #import <Foundation/Foundation.h>
 
-
-//! Project version string for OLMKit, the same as libolm.
-NSString *OLMKitVersionString();
-
 // In this header, you should import all the public headers of your framework using statements like #import <OLMKit/PublicHeader.h>
 
 #import <OLMKit/OLMAccount.h>
@@ -30,3 +26,10 @@ NSString *OLMKitVersionString();
 #import <OLMKit/OLMUtility.h>
 #import <OLMKit/OLMInboundGroupSession.h>
 #import <OLMKit/OLMOutboundGroupSession.h>
+
+@interface OLMKit : NSObject
+
+//! Project version string for OLMKit, the same as libolm.
++ (NSString*)versionString;
+
+@end

xcode/OLMKit/OLMKit.m

@@ -19,7 +19,9 @@
 
 #include "olm/olm.h"
 
-NSString *OLMKitVersionString()
+@implementation OLMKit
+
++ (NSString*)versionString
 {
     uint8_t major, minor, patch;
     
@@ -27,3 +29,5 @@ NSString *OLMKitVersionString()
     
     return [NSString stringWithFormat:@"%tu.%tu.%tu", major, minor, patch];
 }
+
+@end