Fix build tags for PHP5 target

The PHP5 target was broken, since PHP7 builds were not excluded for PHP5-scoped tags.

php7-debian.go

@@ -7,8 +7,7 @@
 // conventions for include and library paths, so we need a specific build tag
 // for building against those packages.
 //
-// +build php7
-// +build debian
+// +build debian,!php5
 
 package php
 

php7.go

@@ -1,6 +1,8 @@
 // Copyright 2017 Alexander Palaistras. All rights reserved.
 // Use of this source code is governed by the MIT license that can be found in
 // the LICENSE file.
+//
+// +build !php5
 
 package php