Add a header file for strings

Add a string.h header for libraries that expect this to be available, now
that U-Boot's version has moved to include/linux.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/string.h b/include/string.h
new file mode 100644
index 0000000..38134dc
--- /dev/null
+++ b/include/string.h
@@ -0,0 +1 @@
+#include <linux/string.h>