cygwin: export __getpagesize

cygwin: export __getpagesize

The inclusion of <sys/cygwin.h> by <sys/shm.h>, besides causing namespace
pollution, also makes it very difficult to get the WINVER-dependent parts
of the former.  This affects code (such as x11vnc -unixpw_nis) which use
both SysV shared memory (e.g. the X11 MIT-SHM extension) and user password
authentication.

getpagesize is the simplest function to retreive this information, but it
is a legacy function and would also pollute the global namespace. The LSB
lists another form which is in the implementation-reserved namespace:

http://refspecs.linuxfoundation.org/LSB_3.1.0/LSB-Core-generic/LSB-Core-generic/baselib---getpagesize.html

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>