kbuild: scripts/gdb: Replace missed $(srctree)/$(src) w/ $(src)

Recently we went through the source tree and replaced
$(srctree)/$(src) w/ $(src). However, the gdb scripts Makefile had a
hidden $(srctree)/$(src) that looked like this:

  $(abspath $(srctree))/$(src)

Because we missed that then my installed kernel had symlinks that
looked like this:

  __init__.py ->
    ${INSTALL_DIR}/$(INSTALL_DIR}/scripts/gdb/linux/__init__.py

Let's also replace the midden $(abspath $(srctree))/$(src) with
$(src). Now:

  __init__.py ->
    $(INSTALL_DIR}/scripts/gdb/linux/__init__.py

Fixes: b1992c37

 ("kbuild: use $(src) instead of $(srctree)/$(src) for source directory")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>