[gtk] README.md: Minor updates

[Matthias Clasen <matthiasc src gnome org>]

commit ab322e33aa1e23f569c4427a6ba55dbcacb265c5
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Feb 24 18:06:51 2018 -0500

    README.md: Minor updates

 README.md | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/README.md b/README.md
index 2a523d0f0a..64aa178098 100644
--- a/README.md
+++ b/README.md
@@ -59,10 +59,10 @@ If you are building the X11 backend, you will also need:
     - xrender
     - xi
     - xext
-    - xfixes (optional)
-    - xcursor (optional)
-    - xdamage (optional)
-    - xcomposite (optional)
+    - xfixes
+    - xcursor
+    - xdamage
+    - xcomposite
   - [atk-bridge-2.0](https://download.gnome.org/sources/at-spi2-atk)
 
 If you are building the Wayland backend, you will also need: