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 <[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/Makefile.config

···
   ifeq ($(feature-bionic), 1)
     BIONIC := 1
     CFLAGS += -DLACKS_SIGQUEUE_PROTOTYPE
+    CFLAGS += -DLACKS_OPEN_MEMSTREAM_PROTOTYPE
     EXTLIBS := $(filter-out -lrt,$(EXTLIBS))
     EXTLIBS := $(filter-out -lpthread,$(EXTLIBS))
   endif

tools/perf/builtin-timechart.c

···
 #include "util/data.h"
 #include "util/debug.h"
 
+#ifdef LACKS_OPEN_MEMSTREAM_PROTOTYPE
+FILE *open_memstream(char **ptr, size_t *sizeloc);
+#endif
+
 #define SUPPORT_OLD_POWER_EVENTS 1
 #define PWR_EVENT_EXIT -1