Guard unsafe imports behind !appengine

"unsafe" (in this case, imported via "golang.org/x/sys/unix") cannot be
used on AppEngine, so we need to add the "!appengine" build constraint
to terminal_check_unix.go.

Fixes #947.

terminal_check_unix.go

@@ -1,4 +1,5 @@
 // +build linux aix
+// +build !appengine
 
 package logrus
 
@@ -10,4 +11,3 @@ func isTerminal(fd int) bool {
 	_, err := unix.IoctlGetTermios(fd, ioctlReadTermios)
 	return err == nil
 }
-