Use the correct include for pthread_self() on the Hurd. (#129)

include/bx/os.h

@@ -49,7 +49,7 @@
 #	elif BX_PLATFORM_OSX
 #		include <mach/mach.h> // mach_task_basic_info
 #	elif BX_PLATFORM_HURD
-#		include <unistd.h> // getpid
+#		include <pthread/pthread.h> // pthread_self
 #	elif BX_PLATFORM_ANDROID
 #		include "debug.h" // getTid is not implemented...
 #	endif // BX_PLATFORM_ANDROID