Make check_files ignore png files in docs

Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/tests/scripts/check_files.py b/tests/scripts/check_files.py
index a2a9dfa..a93b825 100755
--- a/tests/scripts/check_files.py
+++ b/tests/scripts/check_files.py
@@ -105,6 +105,7 @@
 
 BINARY_FILE_PATH_RE_LIST = [
     r'docs/.*\.pdf\Z',
+    r'docs/.*\.png\Z',
     r'programs/fuzz/corpuses/[^.]+\Z',
     r'tests/data_files/[^.]+\Z',
     r'tests/data_files/.*\.(crt|csr|db|der|key|pubkey)\Z',