maintainer-scripts: fix jit docs on website

I noticed whilst working on the libgdiagnostics docs
that some errors like this were occurring in the jit docs:

/tmp/gcc-doc-update.3782849/gcc/gcc/jit/docs/cp/topics/asm.rst:63: WARNING: Include file '/tmp/gcc-doc-update.3782849/gcc/gcc/testsuite/jit.dg/test-asm.cc' not found or reading it failed

which was occurring for:
* test-asm.c and .cc
* test-switch.c
* test-accessing-union.c

and indeed https://gcc.gnu.org/onlinedocs/jit/topics/asm.html

 is
currently missing various code examples.

Fixed thusly; tested locally.

maintainer-scripts/ChangeLog:
	* update_web_docs_git: Add the jit testsuite to the files to
	be preserved, since this is used by the jit docs.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>