mainloop.c - malloc without a prototype

mainloop.c - malloc without a prototype

I noticed a little problem building OpenConnect against gnutls 3;
mainloop.c uses malloc() in queue_new_packet(), somewhere in the chain
of openssl headers stdlib.h gets pulled in so it works ok there, but
this isn't the case with a gnutls build.

Signed-off-by: Stuart Henderson <sthen@openbsd.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>