Added missing include for bool type - stdbool.h

Added include for stdbool.h in php_zmq_private.h. This fixed the type error I was getting while running 'make' under FreeBSD 9.1.

/root/php-zmq/zmq_device.c:70: error: expected declaration specifiers or '...' before 'bool'
/root/php-zmq/zmq_device.c: In function 's_capture_message':
/root/php-zmq/zmq_device.c:85: error: 'more' undeclared (first use in this function)
/root/php-zmq/zmq_device.c:85: error: (Each undeclared identifier is reported only once
/root/php-zmq/zmq_device.c:85: error: for each function it appears in.)
/root/php-zmq/zmq_device.c: In function 'php_zmq_device':
/root/php-zmq/zmq_device.c:179: error: too many arguments to function 's_capture_message'
/root/php-zmq/zmq_device.c:225: error: too many arguments to function 's_capture_message'
*** [zmq_device.lo] Error code 1