[readline] Fix compilation on MinGW

[readline] Fix compilation on MinGW

S_ISLNK should not be called outside of an #ifdef.

However, this specific call is actually unnecessary, because
linkok can only be 0 if S_ISLNK is true, per the code above.

I have sent a bug report to bug-readline, though it does not show
up in the online archive yet.

2019-08-13  Christian Biesinger  <cbiesinger@google.com>

	* colors.c (_rl_print_color_indicator): Remove unnecessary
	S_ISLNK check to fix compilation on MinGW.