fix mp_error_to_string in tommath.h
diff --git a/tommath.h b/tommath.h
index a5fceec..1baff1f 100644
--- a/tommath.h
+++ b/tommath.h
@@ -190,7 +190,7 @@ typedef int ltm_prime_callback(unsigned char *dst, int len, void *dat);
#define SIGN(m) ((m)->sign)
/* error code to char* string */
-char *mp_error_to_string(int code);
+const char *mp_error_to_string(int code);
/* ---> init and deinit bignum functions <--- */
/* init a bignum */