kbuild: fix empty ${PYTHON} in scripts/link-vmlinux.sh

The two commits

  d8d2d382 ("kbuild: remove PYTHON variable")
  a8cccdd9 ("init: lto: ensure initcall ordering")

were applied in the same development cycle, into two different trees.

After they were merged together, this ${PYTHON} expands to an empty
string.

Therefore, ${srctree}/scripts/jobserver-exec is executed directly.
(it has the executable bit set)

This is working but let's fix the code into the intended form.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>

scripts/link-vmlinux.sh

@@ -50,7 +50,7 @@ gen_initcalls()
 {
 	info GEN .tmp_initcalls.lds
 
-	${PYTHON} ${srctree}/scripts/jobserver-exec		\
+	${PYTHON3} ${srctree}/scripts/jobserver-exec		\
 	${PERL} ${srctree}/scripts/generate_initcall_order.pl	\
 		${KBUILD_VMLINUX_OBJS} ${KBUILD_VMLINUX_LIBS}	\
 		> .tmp_initcalls.lds