Explicitly exclude LLVMLibrary from SpotBugs

Not analyzed anyway, because it is too large - and there's nothing
interesting to analyze

Author: lembergerth

build/SpotBugs.exclude.xml

@@ -12,4 +12,5 @@
     <Match><Bug pattern="RV_RETURN_VALUE_IGNORED"/></Match>
     <!-- Exclude AutoValue classes -->
     <Match><Class name="~.*\.AutoValue_.*"/></Match>
+    <Match><Class name="~.*\.LLVMLibrary"/></Match>
 </FindBugsFilter>