mtrace: Add attribute nocommon to mallwatch

Avoid compilation errors GCC versions that do not default to
-fno-common, e.g. GCC <= 9.

Fixes commit 00d28960 ("mtrace:
Deprecate mallwatch and tr_break").

Suggested-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Suggested-by: Florian Weimer <fweimer@redhat.com>