Add newline to avoid broken source files (#3570)

### Motivation:

An upcoming change in the library template adds a code snippet. Since
our cxx interop compatibility test script does not account for that
tests fail on nightly.

### Modifications:

Insert a newline character before other modifications.

### Result:

The cxx interop compatibility tests pass again on nightly.

scripts/check-cxx-interop-compatibility.sh

@@ -37,6 +37,7 @@ swift package init
 } >> Package.swift
 
 echo "package.dependencies.append(.package(path: \"$source_dir\"))" >> Package.swift
+echo >> "$source_file" # Line break to account for template code in the file
 
 for product in $library_products; do
   echo "package.targets.first!.dependencies.append(.product(name: \"$product\", package: \"$package_name\"))" >> Package.swift