bpf: syscall_nrs: Disable no previous prototype warnning

bpf: syscall_nrs: Disable no previous prototype warnning

In some environments (gcc treated as error in W=1, which is default), if we
make -C samples/bpf/, it will be stopped because of
"no previous prototype" error like this:

  ../samples/bpf/syscall_nrs.c:7:6:
  error: no previous prototype for ‘syscall_defines’ [-Werror=missing-prototypes]
   void syscall_defines(void)
        ^~~~~~~~~~~~~~~

Actually, this file meets our expectatations because it will be converted to
a .h file. In this way, it's correct. Considering the warnning stopping us
compiling, we can remove the warnning directly.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/all/20241001012540.39007-1-kerneljasonxing@gmail.com/
Link: https://lore.kernel.org/all/CAEf4BzaVdr_0kQo=+jPLN++PvcU6pwTjaPVEA880kgDN94TZYw@mail.gmail.com/
Link: https://lore.kernel.org/bpf/20241001233242.98679-1-kerneljasonxing@gmail.com