Makefile: add $(srctree) to dependency of compile_commands.json target

When trying to build compile_commands.json for an external module against
the kernel built in a separate output directory, the following error is
displayed:

  make[1]: *** No rule to make target 'scripts/clang-tools/gen_compile_commands.py',
  needed by 'compile_commands.json'. Stop.

This is because gen_compile_commands.py was previously looked up using a
relative path to $(srctree), but commit b1992c37 ("kbuild: use
$(src) instead of $(srctree)/$(src) for source directory") stopped
defining VPATH for external module builds.

Prefixing gen_compile_commands.py with $(srctree) fixes the problem.

Fixes: b1992c37 ("kbuild: use $(src) instead of $(srctree)/$(src) for source directory")
Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>