watch: fix unsetting of COLOR flag

watch used to check if COLOR was required, check if color was
possible then.. set the flag again.

It should have been cleared after failing to get colors out of
ncurses.

References:
 procps-ng/procps#143

watch.c

@@ -806,7 +806,7 @@ int main(int argc, char *argv[])
 			use_default_colors();
 			init_ansi_colors();
 		} else {
-			flags |= WATCH_COLOR;
+			flags &= ~WATCH_COLOR;
 		}
 	}
 	nonl();