Work around format-security false positive (Fixes #9)

Even though the rb_raise call is provably safe (static function
called with all static arguments), format-security still complains.
To appease it and make sequel_pg work in setups where
-Werror=format-security is used by default, use "%s" as the format
string.

CHANGELOG

@@ -1,3 +1,7 @@
+=== HEAD
+
+* Work around format-security false positive (jeremyevans) (#9)
+
 === 1.6.5 (2013-03-06)
 
 * Handle infinite dates using Database#convert_infinite_timestamps (jeremyevans)

ext/sequel_pg/sequel_pg.c

@@ -276,7 +276,7 @@ static VALUE spg_timestamp_error(const char *s, VALUE self, const char *error_ms
       return rb_funcall(db, spg_id_infinite_timestamp_value, 1, rb_tainted_str_new2(s));
     }
   }
-  rb_raise(rb_eArgError, error_msg);
+  rb_raise(rb_eArgError, "%s", error_msg);
 }
 
 static VALUE spg_date(const char *s, VALUE self) {