Add sdl2 package

Note that the package depends on sdl2 >= 2.0.3 via pkg-config.

build-constraints.yaml

@@ -695,6 +695,9 @@ packages:
         - diagrams-rasterific
         - lucid-svg
 
+    "Gabríel Arthúr Pétursson gabriel@system.is":
+        - sdl2
+
     "Stackage upper bounds":
 
         # Force a specific version that's compatible with transformers 0.3