Fix requires

The postgresl control file parser do not add up multiple occurences of the same keyword. 
In practice it's not a big deal as plpgsql is installed by default.

Author: rdunklau

sql/common/pgrouting.control

@@ -3,5 +3,4 @@ comment = 'pgRouting Extension'
 default_version = '${PROJECT_VERSION}'
 module_pathname = '${PROJECT_MODULE_PATHNAME}'
 relocatable = true
-requires = 'plpgsql'
-requires = 'postgis'
+requires = 'plpgsql,postgis'