perf header: Fix up argument to ctime()

Reducing this noise when cross building to the Android NDK:

util/header.c: In function 'perf_header__fprintf_info':
util/header.c:2710:45: warning: pointer targets in passing argument 1 of 'ctime' differ in signedness [-Wpointer-sign]
fprintf(fp, "# captured on : %s", ctime(&st.st_ctime));
^
In file included from util/../perf.h:5:0,
from util/evlist.h:11,
from util/header.c:22:
/opt/android-ndk-r15c/platforms/android-26/arch-arm/usr/include/time.h:81:14: note: expected 'const time_t *' but argument is of type 'long unsigned int *'
extern char* ctime(const time_t*) __LIBC_ABI_PUBLIC__;
^

Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/header.c

···
 	struct perf_header *header = &session->header;
 	int fd = perf_data__fd(session->data);
 	struct stat st;
+	time_t stctime;
 	int ret, bit;
 
 	hd.fp = fp;
···
 	if (ret == -1)
 		return -1;
 
-	fprintf(fp, "# captured on    : %s", ctime(&st.st_ctime));
+	stctime = st.st_ctime;
+	fprintf(fp, "# captured on    : %s", ctime(&stctime));
 
 	fprintf(fp, "# header version : %u\n", header->version);
 	fprintf(fp, "# data offset    : %" PRIu64 "\n", header->data_offset);