scripts: add missing pyyaml dependency

`pyyaml` is a dependency introduced in `imgtool dumpinfo`

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
diff --git a/scripts/requirements.txt b/scripts/requirements.txt
index 2446928..79883b6 100644
--- a/scripts/requirements.txt
+++ b/scripts/requirements.txt
@@ -2,3 +2,4 @@
 intelhex
 click
 cbor2
+pyyaml