perf bpf: Add license(NAME) helper

To further reduce boilerplate.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-vst6hj335s0ebxzqltes3nsc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/examples/bpf/5sec.c

@@ -40,5 +40,4 @@ int func(void *ctx, int err, long sec)
 	return sec == 5;
 }
 
-char _license[] SEC("license") = "GPL";
-int _version SEC("version") = LINUX_VERSION_CODE;
+license(GPL);

tools/perf/examples/bpf/empty.c

@@ -1,4 +1,3 @@
 #include <bpf.h>
 
-char _license[] SEC("license") = "GPL";
-int _version SEC("version") = LINUX_VERSION_CODE;
+license(GPL);