libbpf: Fix -Wdiscarded-qualifiers under C23

commit d70f79f upstream.

glibc ≥ 2.42 (GCC 15) defaults to -std=gnu23, which promotes
-Wdiscarded-qualifiers to an error.

In C23, strstr() and strchr() return "const char *".

Change variable types to const char * where the pointers are never
modified (res, sym_sfx, next_path).

Suggested-by: Florian Weimer <fweimer@redhat.com>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Link: https://lore.kernel.org/r/20251206092825.1471385-1-mikhail.v.gavrilov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
[ shung-hsi.yu: needed to fix kernel build failure due to libbpf since glibc
  2.43+ (which adds 'const' qualifier to strstr). 'sym_sfx' hunk dropped because
  commit f8a0569 ("libbpf: Work around kernel inconsistently stripping
  '.llvm.' suffix") is not present. ]
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: NTMan

tools/lib/bpf/libbpf.c

@@ -8174,7 +8174,7 @@ static int kallsyms_cb(unsigned long long sym_addr, char sym_type,
 	struct bpf_object *obj = ctx;
 	const struct btf_type *t;
 	struct extern_desc *ext;
-	char *res;
+	const char *res;
 
 	res = strstr(sym_name, ".llvm.");
 	if (sym_type == 'd' && res)
@@ -11959,7 +11959,7 @@ static int resolve_full_path(const char *file, char *result, size_t result_sz)
 		if (!search_paths[i])
 			continue;
 		for (s = search_paths[i]; s != NULL; s = strchr(s, ':')) {
-			char *next_path;
+			const char *next_path;
 			int seg_len;
 
 			if (s[0] == ':')