perf riscv: Fix discarded const qualifier in _get_field()

The assignment of strrchr() return values to non-const char * variables
triggers a -Werror=discarded-qualifiers warning when building with GCC
14.

This happens because in newer glibc versions, strrchr() returns a 'const
char *' if the input string is const.

Properly declare 'line2' and 'nl' as const char * to match the glibc
function signature and ensure type safety. This avoids the need for
explicit type casting and aligns with the design pattern of not
modifying read-only memory in the perf tool.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Li Guan <guanli.oerv@isrc.iscas.ac.cn>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <pjw@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/riscv/util/header.c

@@ -19,7 +19,7 @@
 
 static char *_get_field(const char *line)
 {
-	char *line2, *nl;
+	const char *line2, *nl;
 
 	line2 = strrchr(line, ' ');
 	if (!line2)