Build fix: poll compatibility mode: zmalloc prototype changed

Build fix: poll compatibility mode: zmalloc prototype changed

The build fails on platforms that don't support the epoll system
call (or when building with --disable-epoll on Linux):

  compat/poll.cpp:458:35: error: no matching function for call to 'zmalloc'
          wait->events = (struct pollfd *) zmalloc(size * sizeof(struct pollfd));
                                           ^~~~~~~
  ./macros.hpp:85:4: note: candidate template ignored: couldn't infer template argument 'T'
  T *zmalloc(size_t size)
     ^
  ./macros.hpp:74:4: note: candidate function template not viable: requires 0 arguments, but 1 was provided
  T *zmalloc()
     ^
  compat/poll.cpp:466:38: error: no matching function for call to 'zmalloc'
          current->events = (struct pollfd *) zmalloc(size * sizeof(struct pollfd));
                                              ^~~~~~~
  ./macros.hpp:85:4: note: candidate template ignored: couldn't infer template argument 'T'
  T *zmalloc(size_t size)
     ^
  ./macros.hpp:74:4: note: candidate function template not viable: requires 0 arguments, but 1 was provided
  T *zmalloc()

Replace the uses of "old style" malloc with the new type-safe
function introduced recently.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib1660f6a548c155f021843b7476d5d64c06c6e5a