THRIFT-2240 thrift.vim (contrib) does not correctly handle 'union'

Patch: Matt Butcher
diff --git a/contrib/thrift.vim b/contrib/thrift.vim
index 79ce547..3000b46 100644
--- a/contrib/thrift.vim
+++ b/contrib/thrift.vim
@@ -52,7 +52,7 @@
 syn keyword thriftKeyword xsd_all xsd_optional xsd_nillable xsd_attrs
 syn keyword thriftKeyword include cpp_include cpp_type const optional required
 syn keyword thriftBasicTypes void bool byte i16 i32 i64 double string binary
-syn keyword thriftStructure map list set struct typedef exception enum throws
+syn keyword thriftStructure map list set struct typedef exception enum throws union
 
 " Special
 syn match thriftSpecial "\d\+:"