perf header: Fix up argument to ctime()

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 <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-6bz74zp080yhmtiwb36enso9@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>