Only add files in GlobSymbolsDir.

I previously skipped directories, but that can still lead to problems.
So instead, only add paths that are files.

Test: Ran over a stack that failed when trying to run llvm-readelf
Test: on a directory. After this fix, everything get line numbers.
Change-Id: Ide39c5a118d78a822057cc3977d0f532595f798b

scripts/stack_core.py

@@ -319,9 +319,8 @@ class TraceConverter:
   def GlobSymbolsDir(self, symbols_dir):
     files_by_basename = {}
     for path in sorted(pathlib.Path(symbols_dir).glob("**/*")):
-      if os.path.isdir(path):
-        next
-      files_by_basename.setdefault(path.name, []).append(path)
+      if os.path.isfile(path):
+        files_by_basename.setdefault(path.name, []).append(path)
     return files_by_basename
 
   # Use the "file" command line tool to find the bitness and build_id of given ELF file.