TOOLS/matroska.py: support outputting to file

Like the previous commit, it's better to just output it to a file for
meson.

TOOLS/matroska.py

@@ -459,10 +459,15 @@ if __name__ == "__main__":
     def parse_toplevel(s):
         parse_one(s, 0, None, 1 << 63)
 
+    if len(sys.argv) < 3:
+        outfile = sys.stdout
+    else:
+        outfile = open(sys.argv[2], "w")
+
     if sys.argv[1] == '--generate-header':
-        generate_C_header(sys.stdout)
+        generate_C_header(outfile)
     elif sys.argv[1] == '--generate-definitions':
-        generate_C_definitions(sys.stdout)
+        generate_C_definitions(outfile)
     else:
         s = open(sys.argv[1], "rb")
         while 1: