libbpf: fix const correctness in resolve_full_path for gcc15

gcc 15's C library headers use _Generic to return const char * from
strchr() when passed a const char * input. With -Werror=discarded-
qualifiers, assigning this to a char * fails.

next_path only uses the pointer for arithmetic to calculate segment
length, never modifies the string. const char * is the correct type.

Signed-off-by: EvilOlaf <evilolaf@users.noreply.github.com>

Author: EvilOlaf

tools/lib/bpf/libbpf.c

@@ -10738,7 +10738,7 @@ 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] == ':')