imgtool: Fixed missing dependency to 'pyyaml' (for dumpinfo)

imgtool's dumpinfo depends to pyyaml package, so add it to
requirements.

Signed-off-by: Denis Mingulov <denis@mingulov.com>
diff --git a/scripts/setup.py b/scripts/setup.py
index 692cfb7..07d52d1 100644
--- a/scripts/setup.py
+++ b/scripts/setup.py
@@ -18,6 +18,7 @@
         'intelhex>=2.2.1',
         'click',
         'cbor2',
+        'pyyaml',
     ],
     entry_points={
         "console_scripts": ["imgtool=imgtool.main:imgtool"]