config.mak.uname: add arc4random to the cygwin build

config.mak.uname: add arc4random to the cygwin build

The arc4random_buf() function has been available in cygwin since
about 2016 (somewhere in the v2.x branch). Set the CSPRNG_METHOD
build variable to 'arc4random', in the cygwin section, to enable
the use of this cryptographically-secure pseudorandom number
function. Note that the autoconf and new meson builds also enable
this function.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>