shell: add missing blank line after declaration

Fix checkpatch compliance issue by adding a blank line after
declaration.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

subsys/shell/shell.c

@@ -1118,6 +1118,7 @@ static void state_collect(const struct shell *sh)
 				__fallthrough;
 			case 'L': {/* INSERT Button in VT100 mode */
 				bool status = z_flag_insert_mode_get(sh);
+
 				z_flag_insert_mode_set(sh, !status);
 				break;
 			}