Update podspec to support macOS

Merged from https://github.com/aapierce0/OLMKit/commits/macOS_port

Sign-off taken from https://github.com/matrix-org/matrix-ios-sdk/pull/218#issue-201062079.

OLMKit.podspec

@@ -20,7 +20,8 @@ Pod::Spec.new do |s|
   s.authors            = { "Chris Ballinger" => "chrisballinger@gmail.com",
                            "matrix.org" => "support@matrix.org" }
 
-  s.platform     = :ios, "5.0"
+  s.ios.deployment_target = "5.0"
+  s.osx.deployment_target = "10.9"
 
   # Expose the Objective-C wrapper API of libolm
   s.public_header_files = "xcode/OLMKit/*.h"

xcode/OLMKit/OLMKit.h

@@ -16,7 +16,8 @@
  limitations under the License.
  */
 
-#import <UIKit/UIKit.h>
+#import <Foundation/Foundation.h>
+
 
 //! Project version string for OLMKit, the same as libolm.
 NSString *OLMKitVersionString();