http: use xmalloc with cURL

http: use xmalloc with cURL

f0ed8226c9 (Add custom memory allocator to MinGW and MacOS builds, 2009-05-31)
never told cURL about it.

Correct that by using the cURL initializer available since version 7.12 to
point to xmalloc and friends for consistency which then will pass the
allocation requests along when USE_NED_ALLOCATOR=YesPlease is used (most
likely in Windows)

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>