php: Enable prefix testcase

This tests that the SWIG/PHP -prefix command line option works.  It
has a _runme.php and the testcase passes, but it wasn't listed to
be run.
diff --git a/Examples/test-suite/php/Makefile.in b/Examples/test-suite/php/Makefile.in
index 741395a..9f20434 100644
--- a/Examples/test-suite/php/Makefile.in
+++ b/Examples/test-suite/php/Makefile.in
@@ -16,6 +16,7 @@
 	php_iterator \
 	php_namewarn_rename \
 	php_pragma \
+	prefix \
 
 include $(srcdir)/../common.mk