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

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 <irogers@google.com>
Link: https://lore.kernel.org/r/20251211221756.96294-4-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>