Use the GuardedBy import that the Jenkins core uses by convention

- We will never mind that this is probably the wrong import to use as Jenkins core is consistently using the
  wrong one (If anyone asks, for a `javax` import to be correct, it should have a JSR that has passed through voting
  and not have a JSR that is still subject to change... OTOH one could argue that if the JSR 305 should ever
  become revived from its current dormant state *and* it is decided to include in the JRE *then* the packages
  would get moved out of `javax` and into `java` so they are not in final form anyway... but that pre-supposes
  that the process of standardization does not modify the annotations and by virtue of being in `javax`
  there will be some special case handling by some classloaders which is why depending on the JSR305 versions
  is, in my view, a bad plan... anyway this is moot... Jenkins core is going with JSR305 as it currently stands
  so we call all go down together!)