Add <sys/mtio.h>.

In a similar style to some of our other "not really, but enough" headers
like <sys/vt.h>.

Bug: N/A
Test: build GNU dd or BSD dd with a standalone toolchain
Change-Id: I8fbd1aac1d97e24b05e7aae8a55666300b5bf1ed
diff --git a/libc/include/sys/mtio.h b/libc/include/sys/mtio.h
new file mode 100644
index 0000000..8fb5655
--- /dev/null
+++ b/libc/include/sys/mtio.h
@@ -0,0 +1 @@
+#include <linux/mtio.h>