binman: Add coverage to requirements

We need the code-coverage package to run the coverage tests. Add this
package.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/requirements.txt b/tools/binman/requirements.txt
index f068ef7..7db72e8 100644
--- a/tools/binman/requirements.txt
+++ b/tools/binman/requirements.txt
@@ -1,3 +1,4 @@
+coverage==7.8.0
 importlib_resources==6.5.2
 jsonschema==4.23.0
 pycryptodomex==3.21.0