[Process] Fix linux arm64 single step compilation failure

This was updated in r356703 to use llvm::sys::RetryAfterSignal, which
comes from llvm/Support/Errno.h. The header wasn't added, so it fails if
you compile for arm64/aarch64.

llvm-svn: 358530

lldb/source/Plugins/Process/Linux/SingleStepCheck.cpp

@@ -16,6 +16,7 @@
 #include "NativeProcessLinux.h"
 
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/Errno.h"
 
 #include "Plugins/Process/POSIX/ProcessPOSIXLog.h"
 #include "lldb/Host/linux/Ptrace.h"