Update test processor for backing fields

test-utils/src/main/kotlin/com/google/devtools/ksp/processor/AnnotationsInDependenciesProcessor.kt

diff --git a/test-utils/src/main/kotlin/com/google/devtools/ksp/processor/AnnotationsInDependenciesProcessor.kt b/test-utils/src/main/kotlin/com/google/devtools/ksp/processor/AnnotationsInDependenciesProcessor.kt
index e351f73..fd48aa0 100644
--- a/test-utils/src/main/kotlin/com/google/devtools/ksp/processor/AnnotationsInDependenciesProcessor.kt
+++ b/test-utils/src/main/kotlin/com/google/devtools/ksp/processor/AnnotationsInDependenciesProcessor.kt
@@ -55,6 +55,7 @@
             } ?: "no-name-value-parameter ${this.location.lineNumber}"
             is KSPropertyGetter -> "getter of ${receiver.toSignature()}" // lineNumber handled by recursive call
             is KSPropertySetter -> "setter of ${receiver.toSignature()}" // lineNumber handled by recursive call
+            is KSBackingField -> "field of ${property.toSignature()}" // lineNumber handled by recursive call
             else -> {
                 error("unexpected annotated")
             }