Merge pull request pocoproject#691 from cnvandijk/posix-monotonic-clock

Include missing header, fixes pocoproject#690 lack of monotonic Clock on Linux/POSIX

Author: aleks-f

Foundation/src/Clock.cpp

@@ -22,6 +22,7 @@
 #include <mach/clock.h>
 #elif defined(POCO_OS_FAMILY_UNIX)
 #include <time.h>
+#include <unistd.h>
 #elif defined(POCO_VXWORKS)
 #include <timers.h>
 #elif defined(POCO_OS_FAMILY_WINDOWS)