Explicitly cast the return variable in tls_load_file()

[Kinichiro Inoguchi]

I would like to cast the return variable explicitly in tls_load_file().
This fix also avoiding Intel C++ compiler "assertion failed" described here.
https://github.com/libressl-portable/portable/issues/209#issuecomment-249587024

ok ?

Index: tls_util.c
===================================================================
RCS file: /cvs/src/lib/libtls/tls_util.c,v
retrieving revision 1.3
diff -u -p -r1.3 tls_util.c
--- tls_util.c  9 Sep 2015 19:49:07 -0000       1.3
+++ tls_util.c  1 Oct 2016 11:30:28 -0000
@@ -154,7 +154,7 @@ tls_load_file(const char *name, size_t *
 
  done:
        *len = size;
-       return (buf);
+       return ((uint8_t *)buf);
 
  fail:
        free(buf);