Deprecated mallinfo() warning when building

I'm using libc 2.33 and this is now showing up when building tor:

src/lib/meminfo/meminfo.c: In function ‘tor_log_mallinfo’:
src/lib/meminfo/meminfo.c:49:3: warning: ‘mallinfo’ is deprecated [-Wdeprecated-declarations]
   49 |   mi = mallinfo();
      |   ^~
In file included from src/lib/meminfo/meminfo.c:25:
/usr/include/malloc.h:118:24: note: declared here
  118 | extern struct mallinfo mallinfo (void) __THROW __MALLOC_DEPRECATED;
      |                        ^~~~~~~~

... which is a bit annoying when building with enable-gcc-warnings that makes it fatal.

Likely reality that we might want to backport this to all supported versions if we want to be able to build them with libc >= 2.33.