Add filename headers to toc output

Format:
  lib/filename.c:
  [start-end] declaration;

  lib/filename.h:
  [start-end] typedef struct { ... } name;

c_util

@@ -647,6 +647,9 @@ def main():
                 print()
             first_file = False
             
+            # Print filename header
+            print(f"{f}:")
+            
             # Print in new format
             for start, end, text in declarations:
                 print(f"[{start + 1}-{end + 1}] {text}")