Fix compile warnings about ServerBaseOptions
Fix compile warnings about ServerBaseOptions

clang complained that ServerBaseOptions is forward declared
as a class when it's a struct:

[...]kudu/src/server/server_base_options.h:19:8: note: previous use is here
struct ServerBaseOptions {
       ^
[...]kudu/src/server/server_base.h:32:1: note: did you mean struct here?
class ServerBaseOptions;

Fixed this by changing the forward declaration.

Change-Id: If987f7f7d5004a8a7e49c3211c2f20162b546e12
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1490
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Tested-by: jenkins

InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu