os_process.c: Add cygwin as an expected platform

os_process.c: Add cygwin as an expected platform

mesa/mesa/src/gallium/auxiliary/os/os_process.c:40:2: warning: #warning unexpected platform in os_process.c [-Wcpp]
 #warning unexpected platform in os_process.c
mesa/mesa/src/gallium/auxiliary/os/os_process.c:77:2: warning: #warning unexpected platform in os_process.c [-Wcpp]
 #warning unexpected platform in os_process.c

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Brian Paul <brianp@vmware.com>