patch 8.2.0652: compiler warning for char conversion

Problem:    Compiler warning for char conversion.
Solution:   Use unsigned char buffer.
diff --git a/src/userfunc.c b/src/userfunc.c
index 993697e..86dea79 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1586,13 +1586,13 @@
     hashitem_T	*hi;
     ufunc_T	*fp;
     long_u	todo;
-    char	buf[30];
+    char_u	buf[30];
     size_t	len;
 
     buf[0] = K_SPECIAL;
     buf[1] = KS_EXTRA;
     buf[2] = (int)KE_SNR;
-    sprintf(buf + 3, "%d_", sid);
+    sprintf((char *)buf + 3, "%d_", sid);
     len = STRLEN(buf);
 
     todo = func_hashtab.ht_used;