[BUILD] Fix missing dependency on protoc compiler (open-telemetry#3159)

Author: marcalff

cmake/opentelemetry-proto.cmake

@@ -288,7 +288,9 @@ add_custom_command(
     ${LOGS_PROTO} ${METRICS_PROTO} ${TRACE_SERVICE_PROTO} ${LOGS_SERVICE_PROTO}
     ${METRICS_SERVICE_PROTO} ${PROFILES_PROTO}
     ${PROFILES_SERVICE_PROTO}
-  COMMENT "[Run]: ${PROTOBUF_RUN_PROTOC_COMMAND}")
+  COMMENT "[Run]: ${PROTOBUF_RUN_PROTOC_COMMAND}"
+  DEPENDS ${PROTOBUF_PROTOC_EXECUTABLE}
+  )
 
 include_directories("${GENERATED_PROTOBUF_PATH}")