x86/boot: Mark prepare_command_line() __init

Fix:

  WARNING: modpost: vmlinux.o(.text.unlikely+0x64d0): Section mismatch in reference \
   from the function prepare_command_line() to the variable .init.data:command_line
  The function prepare_command_line() references
  the variable __initdata command_line.
  This is often because prepare_command_line lacks a __initdata
  annotation or the annotation of command_line is wrong.

Apparently some toolchains do different inlining decisions.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/YZySgpmBcNNM2qca@zn.tnic

arch/x86/kernel/setup.c

@@ -742,7 +742,7 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
 	return 0;
 }
 
-static char *prepare_command_line(void)
+static char * __init prepare_command_line(void)
 {
 #ifdef CONFIG_CMDLINE_BOOL
 #ifdef CONFIG_CMDLINE_OVERRIDE