config.mak.uname: remove SPARSE_FLAGS setting for cygwin

config.mak.uname: remove SPARSE_FLAGS setting for cygwin

Since commit f66450ae9 ("cygwin: Remove the Win32 l/stat() implementation",
2013-06-22), the cygwin build has not used the WIN32 API/header files.
This means that the '-isystem /usr/include/w32api' option to sparse is
no longer necessary (to allow sparse to find the WIN32 header files).
In addition, the '-Wno-one-bit-signed-bitfield' option can be removed,
since the warning suppressed by that option was only provoked by a WIN32
header file.

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