xplanet: fix cspice optional dependency.

Closes Homebrew/homebrew#28123.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/xplanet.rb

@@ -27,11 +27,8 @@ class Xplanet < Formula
   depends_on "libtiff" => :recommended
 
   depends_on "netpbm" if build.with?("netpbm") || build.with?("all")
-
-  if build.with?("pango") || build.with?("all")
-    depends_on "pango"
-    depends_on "cspice"
-  end
+  depends_on "pango" if build.with?("pango") || build.with?("all")
+  depends_on "cspice" if build.with?("cspice") || build.with?("all")
 
   depends_on :freetype
   depends_on :x11