x86, kexec: fix the wrong ifdeffery CONFIG_KEXEC

With the current ifdeffery CONFIG_KEXEC, get_cmdline_acpi_rsdp() is only
available when kexec_load interface is taken, while kexec_file_load
interface can't make use of it.

Now change it to CONFIG_KEXEC_CORE.

Link: https://lkml.kernel.org/r/20231208073036.7884-6-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Eric DeVolder <eric_devolder@yahoo.com>
Cc: Ignat Korchagin <ignat@cloudflare.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Baoquan He

arch/x86/boot/compressed/acpi.c

@@ -178,7 +178,7 @@ static unsigned long get_cmdline_acpi_rsdp(void)
 {
 	unsigned long addr = 0;
 
-#ifdef CONFIG_KEXEC
+#ifdef CONFIG_KEXEC_CORE
 	char val[MAX_ADDR_LEN] = { };
 	int ret;