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

JIRA: https://issues.redhat.com/browse/RHEL-107194

commit 6fc9aac
Author: Alexandre Courbot <gnurou@gmail.com>
Date:   Sun Aug 4 14:50:57 2024 +0900

    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 b1992c3 ("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: b1992c3 ("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>

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>

Author: Herton R. Krzesinski

Makefile

@@ -2008,7 +2008,7 @@ nsdeps: modules
 quiet_cmd_gen_compile_commands = GEN     $@
       cmd_gen_compile_commands = $(PYTHON3) $< -a $(AR) -o $@ $(filter-out $<, $(real-prereqs))
 
-$(extmod_prefix)compile_commands.json: scripts/clang-tools/gen_compile_commands.py \
+$(extmod_prefix)compile_commands.json: $(srctree)/scripts/clang-tools/gen_compile_commands.py \
 	$(if $(KBUILD_EXTMOD),,$(KBUILD_VMLINUX_OBJS) $(KBUILD_VMLINUX_LIBS)) \
 	$(if $(CONFIG_MODULES), $(MODORDER)) FORCE
 	$(call if_changed,gen_compile_commands)