perf tools: Add missing open_memstream() prototype for systems lacking it

perf tools: Add missing open_memstream() prototype for systems lacking it

There are systems such as the Android NDK API level 24 has the
open_memstream() function but doesn't provide a prototype, adding noise
to the build:

  builtin-timechart.c: In function 'cat_backtrace':
  builtin-timechart.c:486:2: warning: implicit declaration of function 'open_memstream' [-Wimplicit-function-declaration]
    FILE *f = open_memstream(&p, &p_len);
    ^
  builtin-timechart.c:486:2: warning: nested extern declaration of 'open_memstream' [-Wnested-externs]
  builtin-timechart.c:486:12: warning: initialization makes pointer from integer without a cast
    FILE *f = open_memstream(&p, &p_len);
              ^

Define a LACKS_OPEN_MEMSTREAM_PROTOTYPE define so that code needing that
can get a prototype.

Checked in the bionic git repo to be available since level 23:

https://android.googlesource.com/platform/bionic/+/master/libc/include/stdio.h#241

  FILE* open_memstream(char** __ptr, size_t* __size_ptr) __INTRODUCED_IN(23);

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-343ashae97e5bq6vizusyfno@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>