make: remove the .extradep file in libxfs on "make clean"

make: remove the .extradep file in libxfs on "make clean"

Commit 6e1d3517d108 ("libxfs: test compiling public headers with a C++
compiler") will create the .extradep file.  This can cause future
builds to fail if the header files in $(DESTDIR) no longer exist.

Fix this by removing .extradep (along with files like .ltdep) on a
"make clean".

Fixes: 6e1d3517d108 ("libxfs: test compiling public headers with a C++ compiler")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>