init: fix -Wmissing-variable-declarations warning

Hi all,

I was looking to get some help on solving this -Wmissing-variable-declarations
warning as there is some hope to turn it on for W=1 soon [1].

When building x86/defconfig with Clang-18 I encounter the following warning:
| init/main.c:189:13: warning: no previous extern declaration for non-static variable 'envp_init' [-Wmissing-variable-declarations]
|   189 | const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
|       |             ^
| init/main.c:189:7: note: declare 'static' if the variable is not intended to be used outside of this translation unit
|   189 | const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
|       |       ^

It seems like the obvious solution is to just add the `static` keyword
and be done with it. I suspect, however, that it is not so simple for
the following reasons:

Firstly, `envp_init` is surrounded by two other variables that have been
explicitly marked as `static` which leads me to believe that this one
was intentionally _not_ marked as static for some reason:
| static const char *argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };
| static const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
| static const char *panic_later, *panic_param;

Secondly, there exists this `extern` declaration for `envp_init`:
| init/do_mounts_initrd.c
| 90:     extern char *envp_init[];

This one is tricky because it seems like I can rename (effectively
remove) this extern symbol entirely and still build a kernel image.

If it truly is the case that this extern declaration works then why does
Clang produce the warning at all?

FWIW, I've tried moving `extern char *envp_init[];` to top-level scope
inside of `do_mounts_initrd.c` which did _not_ work in fixing the
warning.

So all in all, it looks like just adding `static` fixes the warning
(which it does) but I am not sure about the other ramifications of this
patch especially considering the second point I brought up above
regarding the extern declaration already existing (but seemingly not
doing anything).

Any help here would be appreciated!

Link: ClangBuiltLinux#1920 [1]
Signed-off-by: Justin Stitt <justinstitt@google.com>

init/main.c

@@ -186,7 +186,7 @@ static int __init set_reset_devices(char *str)
 __setup("reset_devices", set_reset_devices);
 
 static const char *argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };
-const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
+static const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
 static const char *panic_later, *panic_param;
 
 static bool __init obsolete_checksetup(char *line)