boards: cogip-native: mallinfo2 is not supported on Ubuntu 20.04

[ecourtois]

With the introduction of sys/sysmon in a previous release (c1ed38b), applications/examples does not compile anymore with cogip-native board.
It is due to the use of mallinfo2 which was introduced in glibc 2.33, but Ubuntu 20.04 uses glibc 2.31.

In file included from <command-line>:
mcu-firmware/boards/cogip-native/board_init.c: In function 'board_init':
mcu-firmware/examples/sys_sysmon/bin/cogip-native/riotbuild/riotbuild.h:14:25: error: variable 'minfo' has initializer but incomplete type
   14 | #define MALLINFO struct mallinfo2
      |                         ^~~~~~~~~
mcu-firmware/boards/cogip-native/board_init.c:44:5: note: in expansion of macro 'MALLINFO'
   44 |     MALLINFO minfo = MALLINFO_FUNC();
      |     ^~~~~~~~
mcu-firmware/examples/sys_sysmon/bin/cogip-native/riotbuild/riotbuild.h:15:25: error: implicit declaration of function 'mallinfo2'; did you mean 'mallinfo'? [-Werror=implicit-function-declaration]
   15 | #define MALLINFO_FUNC() mallinfo2()
      |                         ^~~~~~~~~
mcu-firmware/boards/cogip-native/board_init.c:44:22: note: in expansion of macro 'MALLINFO_FUNC'
   44 |     MALLINFO minfo = MALLINFO_FUNC();
      |                      ^~~~~~~~~~~~~
mcu-firmware/boards/cogip-native/board_init.c:44:14: error: storage size of 'minfo' isn't known
   44 |     MALLINFO minfo = MALLINFO_FUNC();
      |              ^~~~~
mcu-firmware/boards/cogip-native/board_init.c:44:14: error: unused variable 'minfo' [-Werror=unused-variable]
cc1: all warnings being treated as errors