gdb: Fix copyright year in gdb/testsuite/gdb.base/inferior-clone.exp

I just realized that I forgot to update the year before pushing the
patch that created this file.  Since it landed after the global
copyright year update have been done, this file’s copyright year is
updated.

This patch fixes that.

Change-Id: I280f7d86e02d38425f7afdcf19a1c3500d51c23f