smtp: fix compile warning

app-layer-smtp.c:566:14: error: initializing 'uint8_t *' (aka 'unsigned char *') with an expression of type 'const void *' discards qualifiers
      [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  566 |     uint8_t *lf_idx = memchr(input->buf + input->consumed, 0x0a, input->len);
      |              ^        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

(cherry picked from commit 5aee5ff6ed)

src/app-layer-smtp.c

@@ -663,7 +663,7 @@ static AppLayerResult SMTPGetLine(
     if (input->len <= 0)
         return APP_LAYER_ERROR;
 
-    uint8_t *lf_idx = memchr(input->buf + input->consumed, 0x0a, input->len);
+    const uint8_t *lf_idx = memchr(input->buf + input->consumed, 0x0a, input->len);
     bool discard_till_lf = (direction == 0) ? state->discard_till_lf_ts : state->discard_till_lf_tc;
 
     if (lf_idx == NULL) {