gdb/doc: fix cannot create directory error when building dvi/pdf

After this commit:

  commit 0700386f (gdb-tmp-c)
  Date:   Wed May 8 19:12:57 2024 +0100

      gdb/doc: fix parallel build of pdf and dvi files

When building the dvi or pdf targets you'd get errors like this:

  mkdir: cannot create directory ‘texi2dvi_tmpdir/gdb_dvi’: No such file or directory
  mkdir: cannot create directory ‘texi2dvi_tmpdir/gdb_pdf’: No such file or directory

fixed by ensuring the directory is created before calling texi2dvi.