perf header: Use last modification time for timestamp

perf header: Use last modification time for timestamp

Using .st_ctime clobbers the timestamp information in perf report header
whenever any operation is done with the file. Even tar-ing and untar-ing
the perf.data file (which preserves the file last modification timestamp)
doesn't prevent that:

    [Michael@Diego tmp]$ ls -l perf.data
->	-rw-------. 1 Michael Michael 169888 Dec  2 15:23 perf.data

	[Michael@Diego tmp]$ perf report --header-only
	# ========
->	# captured on    : Mon Dec  2 15:23:42 2019
	 [...]

	[Michael@Diego tmp]$ tar c perf.data | xz > perf.data.tar.xz
	[Michael@Diego tmp]$ mkdir aaa
	[Michael@Diego tmp]$ cd aaa
	[Michael@Diego aaa]$ xzcat ../perf.data.tar.xz | tar x
	[Michael@Diego aaa]$ ls -l -a
	total 172
	drwxrwxr-x. 2 Michael Michael     23 Jan 14 11:26 .
	drwxrwxr-x. 6 Michael Michael   4096 Jan 14 11:26 ..
->	-rw-------. 1 Michael Michael 169888 Dec  2 15:23 perf.data

	[Michael@Diego aaa]$ perf report --header-only
	# ========
->	# captured on    : Tue Jan 14 11:26:16 2020
	 [...]

When using .st_mtime instead, correct information is printed:

	[Michael@Diego aaa]$ ~/acme/tools/perf/perf report --header-only
	# ========
->	# captured on    : Mon Dec  2 15:23:42 2019
	 [...]

Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
LPU-Reference: 20200114104236.31555-1-mpetlan@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>