libc: add EILSEQ for use in reporting checksum failure.

This code is commonly used in the Linux kernel for reporting a
retryable error like a failed CRC.  This name and value is already
present in Linux and newlib.

Signed-off-by: Michael Hope <mlhx@google.com>

lib/libc/minimal/include/errno.h

@@ -126,6 +126,8 @@ extern int *__errno(void);
 #define ETIME 79   /* STREAMS timeout occurred */
 #define ENOMSG 80  /* Unexpected message type */
 
+#define EILSEQ 138 /* Illegal byte sequence */
+
 #ifdef __cplusplus
 }
 #endif