patch 8.2.4402: missing parenthesis may cause unexpected problems

Problem:    Missing parenthesis may cause unexpected problems.
Solution:   Add more parenthesis is macros. (closes #9788)
diff --git a/src/normal.c b/src/normal.c
index 449f841..9376be5 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -1563,7 +1563,7 @@
  * Routines for displaying a partly typed command
  */
 
-#define SHOWCMD_BUFLEN SHOWCMD_COLS + 1 + 30
+#define SHOWCMD_BUFLEN (SHOWCMD_COLS + 1 + 30)
 static char_u	showcmd_buf[SHOWCMD_BUFLEN];
 static char_u	old_showcmd_buf[SHOWCMD_BUFLEN];  // For push_showcmd()
 static int	showcmd_is_clear = TRUE;