fix: Dockerfile.build missing cmake in rwkv example (#301)

Signed-off-by: Tyler Gillson <tyler.gillson@gmail.com>

examples/rwkv/.gitignore

@@ -0,0 +1,2 @@
+models/rwkv
+models/rwkv.tokenizer.json

examples/rwkv/Dockerfile.build

@@ -1,5 +1,7 @@
 FROM python
 
+RUN apt-get update && apt-get -y install cmake
+
 # convert the model (one-off)
 RUN pip3 install torch numpy