Set build for distribution flag

This ensures that the framework generates the .swiftinterface files so
that future compilers are able to load the framework, making it module
stable.

Explained in WWDC 2019: https://developer.apple.com/videos/play/wwdc2019/416/

Reachability.xcodeproj/project.pbxproj

@@ -826,6 +826,7 @@
 			buildSettings = {
 				ALWAYS_EMBED_SWIFT_STANDARD_LIBRARIES = NO;
 				APPLICATION_EXTENSION_API_ONLY = YES;
+				BUILD_LIBRARY_FOR_DISTRIBUTION = YES;
 				CLANG_ENABLE_MODULES = YES;
 				CODE_SIGN_IDENTITY = "";
 				"CODE_SIGN_IDENTITY[sdk=iphoneos*]" = "";
@@ -852,6 +853,7 @@
 			buildSettings = {
 				ALWAYS_EMBED_SWIFT_STANDARD_LIBRARIES = NO;
 				APPLICATION_EXTENSION_API_ONLY = YES;
+				BUILD_LIBRARY_FOR_DISTRIBUTION = YES;
 				CLANG_ENABLE_CODE_COVERAGE = NO;
 				CLANG_ENABLE_MODULES = YES;
 				CODE_SIGN_IDENTITY = "";