libbpf: Remove extern declaration of bpf_stream_vprintk()

libbpf: Remove extern declaration of bpf_stream_vprintk()

An issue was reported that building BPF program which includes both
vmlinux.h and bpf_helpers.h from libbpf fails due to conflicting
declarations of bpf_stream_vprintk().

Remove the extern declaration from bpf_helpers.h to address this.

In order to use bpf_stream_printk() macro, BPF programs are expected
to either include vmlinux.h of the kernel they are targeting, or add
their own extern declaration.

Reported-by: Luca Boccassi <luca.boccassi@gmail.com>
Closes: https://github.com/libbpf/libbpf/issues/947
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260218215651.2057673-3-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>