add static-assert to sex

Expands to C11 _Static_assert keyword

fmt-c-writer.scm

@@ -114,6 +114,7 @@ forms, and what remains."
     ((attribute) '%attribute)
     ((¤) 'vector-ref)
     ((include) '%include)
+    ((static-assert) '_Static_assert)
     ;; a `|' inside a symbol has to be escaped to be written in
     ;; a Scheme source, so we just rename it in fmt-c compatible
     ;; way