kbuild: check the presence of include/generated/rustc_cfg

Since commit 2f7ab12 ("Kbuild: add Rust support"), Kconfig
generates include/generated/rustc_cfg, but its presence is not checked
in the top-level Makefile. It should be checked similarly to the C
header counterpart, include/generated/autoconf.h.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Tested-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -783,7 +783,7 @@ else # !may-sync-config
 # and include/config/auto.conf but do not care if they are up-to-date.
 # Use auto.conf to show the error message
 
-checked-configs := include/generated/autoconf.h include/config/auto.conf
+checked-configs := include/generated/autoconf.h include/generated/rustc_cfg include/config/auto.conf
 missing-configs := $(filter-out $(wildcard $(checked-configs)), $(checked-configs))
 
 ifdef missing-configs
@@ -1201,7 +1201,8 @@ PHONY += prepare archprepare
 
 archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
 	asm-generic $(version_h) include/generated/utsrelease.h \
-	include/generated/compile.h include/generated/autoconf.h remove-stale-files
+	include/generated/compile.h include/generated/autoconf.h \
+	include/generated/rustc_cfg remove-stale-files
 
 prepare0: archprepare
 	$(Q)$(MAKE) $(build)=scripts/mod