gdb/ChangeLog: Add PR number to the latest entry

This adds the PR number to the following gdb/ChangeLog  entry:

2020-05-17  Tom Tromey  <tromey@adacore.com>

    Pushed by Joel Brobecker  <brobecker@adacore.com>
    PR symtab/26003
    * symfile.c (symbol_file_add_separate): Preserve OBJF_MAINLINE.