Fix AppDistro spm build for non-iOS platforms (#7091)

Author: ryanwilson

FirebaseAppDistribution/Sources/FIRAppDistribution.m

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 #import <Foundation/Foundation.h>
+#import <TargetConditionals.h>
+#if TARGET_OS_IOS
 
 #import <GoogleUtilities/GULAppDelegateSwizzler.h>
 #import <GoogleUtilities/GULUserDefaults.h>
@@ -333,3 +335,5 @@ - (BOOL)isCodeHashIdentical:(NSString *)codeHash {
   return codeHash && [codeHash isEqualToString:[machO codeHash]];
 }
 @end
+
+#endif

FirebaseAppDistribution/Sources/FIRAppDistributionUIService.h

@@ -12,13 +12,18 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#import <TargetConditionals.h>
+#if TARGET_OS_IOS
+
 #import <AuthenticationServices/AuthenticationServices.h>
 #import <Foundation/Foundation.h>
 #import <SafariServices/SafariServices.h>
 #import <UIKit/UIKit.h>
 
 #import "FirebaseAppDistribution/Sources/Private/FIRAppDistribution.h"
 
+@protocol SFSafariViewControllerDelegate;
+
 NS_ASSUME_NONNULL_BEGIN
 
 /**
@@ -60,3 +65,5 @@ typedef void (^AppDistributionRegistrationFlowCompletion)(NSError *_Nullable err
 @end
 
 NS_ASSUME_NONNULL_END
+
+#endif

FirebaseAppDistribution/Sources/FIRAppDistributionUIService.m

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#import <TargetConditionals.h>
+#if TARGET_OS_IOS
+
 #import "FirebaseAppDistribution/Sources/FIRAppDistributionUIService.h"
 #import "FirebaseAppDistribution/Sources/FIRFADLogger.h"
 #import "FirebaseAppDistribution/Sources/Public/FirebaseAppDistribution/FIRAppDistribution.h"
@@ -264,3 +267,5 @@ - (ASPresentationAnchor)presentationAnchorForWebAuthenticationSession:
 }
 
 @end
+
+#endif