perf tools: Use const for variables receiving str{str,r?chr}() returns

Newer glibc versions return const char for str{str,chr}() where the
haystack/s is const so to avoid warnings like these on fedora 44 change
some variables to const:

36 8.17 fedora:44 : FAIL gcc version 15.2.1 20251111 (Red Hat 15.2.1-4) (GCC)
libbpf.c: In function 'kallsyms_cb':
libbpf.c:8489:13: error: assignment discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
8489 | res = strstr(sym_name, ".llvm.");

Reviewed-by: Ian Rogers <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/jvmti/libjvmti.c

···
 	*/
 	if (*class_sign == 'L') {
 		size_t j, i = 0;
-		char *p = strrchr(class_sign, '/');
+		const char *p = strrchr(class_sign, '/');
 		if (p) {
 			/* drop the 'L' prefix and copy up to the final '/' */
 			for (i = 0; i < (size_t)(p - class_sign); i++)

tools/perf/tests/parse-events.c

···
 	for (int i = 0; i < cnt; i++) {
 		struct evlist_test e = events[i];
 		int test_ret;
-		const char *pos = e.name;
-		char buf[1024], *buf_pos = buf, *end;
+		const char *pos = e.name, *end;
+		char buf[1024], *buf_pos = buf;
 
 		while ((end = strstr(pos, "default_core"))) {
 			size_t len = end - pos;

tools/perf/util/evlist.c

···
 
 int evlist__parse_control(const char *str, int *ctl_fd, int *ctl_fd_ack, bool *ctl_fd_close)
 {
-	char *comma = NULL, *endptr = NULL;
+	const char *comma = NULL;
+	char *endptr = NULL;
 
 	*ctl_fd_close = false;