kbuild: do not add $(srctree) or $(objtree) to header search paths

scripts/Makefile.lib is included not only from scripts/Makefile.build
but also from scripts/Makefile.{vmlinux,modfinal} for building generated
C files.

In scripts/Makefile.{vmlinux,modfinal}, $(obj) and $(src) are empty.

Therefore, the header include paths:

    -I $(srctree)/$(src) -I $(objtree)/$(obj)

... become meaningless code:

    -I $(srctree)/ -I $(objtree)/

Add these paths only when 'obj' and 'src' are defined.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202404170634.BlqTaYA0-lkp@intel.com/
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>