When we moved zalloc.o to the library we missed gtk library which needs
it compiled in, otherwise the missing __zfree symbol will cause the
library to fail to load.

Adding the zalloc object to the gtk library build.

Fixes: 7f7c536f ("tools lib: Adopt zalloc()/zfree() from tools/perf")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jelle van der Waa <jelle@vdwaa.nl>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200113104358.123511-1-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>