Make UndertowAccessLogCustomizer conditional on class

[mbhave]

[nishantraut]

Hi @mbhave this looks simple change for newbie. i would like to work on this. can you please guide. Thanks

[philwebb]

@nishantraut This should be as simple as adding @ConditionalOnClass(Undertow.class) to this method

[nishantraut]

Thanks Phil will raise PR shortly.

[nishantraut]

@philwebb please review PR and merge if looks ok. Thank you for help and support.