main: Use mallinfo2 when available at build time

`mallinfo` has been deprecated in favor of `mallinfo2`:
```
The fields of the mallinfo structure that is returned by the
older mallinfo() function are typed as int.  However, because
some internal bookkeeping values may be of type long, the
reported values may wrap around zero and thus be inaccurate.
```

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1786>

src/main.c

@@ -2,7 +2,7 @@
 
 #include "config.h"
 
-#ifdef HAVE_MALLINFO
+#if defined (HAVE_MALLINFO) || defined (HAVE_MALLINFO2)
 #include <malloc.h>
 #endif
 #include <stdlib.h>
@@ -212,8 +212,12 @@ static void
 malloc_statistics_callback (ShellPerfLog *perf_log,
                             gpointer      data)
 {
-#ifdef HAVE_MALLINFO
+#if defined (HAVE_MALLINFO) || defined (HAVE_MALLINFO2)
+#ifdef HAVE_MALLINFO2
+  struct mallinfo2 info = mallinfo2 ();
+#else
   struct mallinfo info = mallinfo ();
+#endif
 
   shell_perf_log_update_statistic_i (perf_log,
                                      "malloc.arenaSize",
@@ -224,7 +228,7 @@ malloc_statistics_callback (ShellPerfLog *perf_log,
   shell_perf_log_update_statistic_i (perf_log,
                                      "malloc.usedSize",
                                      info.uordblks);
-#endif
+#endif /* defined (HAVE_MALLINFO) || defined (HAVE_MALLINFO2) */
 }
 
 static void

src/shell-global.c

@@ -31,11 +31,6 @@
 #define GNOME_DESKTOP_USE_UNSTABLE_API
 #include <libgnome-desktop/gnome-systemd.h>
 
-/* Memory report bits */
-#ifdef HAVE_MALLINFO
-#include <malloc.h>
-#endif
-
 #if defined __OpenBSD__ || defined __FreeBSD__
 #include <sys/sysctl.h>
 #endif