Add a symlink for ctype.h

Libraries that link with U-Boot typically may expect to be able to include
ctype.h but this file is in a different place in U-Boot. Add a symlink to
make this work.

Signed-off-by: Simon Glass <sjg@chromium.org>

diff --git a/include/ctype.h b/include/ctype.h
new file mode 120000 (symlink)
index 0000000..9e43f9c
--- /dev/null
+++ b/include/ctype.h
@@ -0,0 +1 @@
+linux/ctype.h
\ No newline at end of file