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: libbpf/libbpf#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>

Author: theihor

tools/lib/bpf/bpf_helpers.h

@@ -315,9 +315,6 @@ enum libbpf_tristate {
 			  ___param, sizeof(___param));		\
 })
 
-extern int bpf_stream_vprintk(int stream_id, const char *fmt__str, const void *args,
-			      __u32 len__sz) __weak __ksym;
-
 #define bpf_stream_printk(stream_id, fmt, args...)					\
 ({											\
 	static const char ___fmt[] = fmt;						\