[SourceKit] Add missing std::string include

`SwiftInvocation.h` references `std::string`, but does not include
`<string>`. This results in an error when compiling on Linux.

tools/SourceKit/lib/SwiftLang/SwiftInvocation.h

@@ -14,6 +14,7 @@
 #define LLVM_SOURCEKIT_LIB_SWIFTLANG_SWIFTINVOCATION_H
 
 #include "swift/Basic/ThreadSafeRefCounted.h"
+#include <string>
 #include <vector>
 
 namespace swift {