init: fix build warnings about export.h

After commit a934a57 ("scripts/misc-check: check missing #include
<linux/export.h> when W=1") and 7d95680 ("scripts/misc-check:
check unnecessary #include <linux/export.h> when W=1"), we get some build
warnings with W=1:

init/main.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing
init/initramfs.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing

So fix these build warnings for the init code.

Link: https://lkml.kernel.org/r/20250608141235.155206-1-chenhuacai@loongson.cn
Fixes: a934a57 ("scripts/misc-check: check missing #include <linux/export.h> when W=1")
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: chenhuacai

init/initramfs.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/init.h>
 #include <linux/async.h>
+#include <linux/export.h>
 #include <linux/fs.h>
 #include <linux/slab.h>
 #include <linux/types.h>

init/main.c

@@ -13,6 +13,7 @@
 #define DEBUG		/* Enable initcall_debug */
 
 #include <linux/types.h>
+#include <linux/export.h>
 #include <linux/extable.h>
 #include <linux/module.h>
 #include <linux/proc_fs.h>