config.mak.uname: add clock_gettime() to the cygwin build

Cygwin supports the clock_gettime() function, along with the associated CLOCK_MONOTONIC preprocessor symbol. The autoconf and meson builds both enable the use of those symbols. In order to have the same configuration for the make builds, add the HAVE_CLOCK_GETTIME and HAVE_CLOCK_MONOTONIC build variables to the cygwin section of the config.mak.uname file. Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>