mallinfo is only defined on glibc and android (#764)

It currently prevents compilation under musl:

```
[ 90%] Building CXX object src/tbbmalloc_proxy/CMakeFiles/tbbmalloc_proxy.dir/proxy.cpp.o
/__w/mimalloc-bench/mimalloc-bench/extern/tbb/src/tbbmalloc_proxy/proxy.cpp:263:26: error: return type 'struct mallinfo' is incomplete
  263 | struct mallinfo mallinfo() __THROW
      |                          ^
compilation terminated due to -Wfatal-errors.
``

src/tbbmalloc_proxy/proxy.cpp

@@ -260,13 +260,15 @@ int mallopt(int /*param*/, int /*value*/) __THROW
     return 1;
 }
 
+#if defined(__GLIBC__) || defined(__ANDROID__)
 struct mallinfo mallinfo() __THROW
 {
     struct mallinfo m;
     memset(&m, 0, sizeof(struct mallinfo));
 
     return m;
 }
+#endif
 
 #if __ANDROID__
 // Android doesn't have malloc_usable_size, provide it to be compatible