x86/syscalls: Fix -Wmissing-prototypes warnings from COND_SYSCALL()

Building kernel/sys_ni.c with W=1 emits tons of -Wmissing-prototypes warnings:

  $ make W=1 kernel/sys_ni.o
    [ snip ]
    CC      kernel/sys_ni.o
     ./arch/x86/include/asm/syscall_wrapper.h:83:14: warning: no previous prototype for '__ia32_sys_io_setup' [-Wmissing-prototypes]
     ...

The problem is in __COND_SYSCALL(), the __SYS_STUB0() and __SYS_STUBx() macros
defined a few lines above already have forward declarations.

Let's do likewise for __COND_SYSCALL() to fix the warnings.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210301131533.64671-2-masahiroy@kernel.org