Fix cross-compiling issue

Author: ruslo

cmake/protobuf-module.cmake.in

@@ -136,9 +136,13 @@ _protobuf_find_libraries(Protobuf protobuf)
 # The Protobuf Lite library
 _protobuf_find_libraries(Protobuf_LITE protobuf-lite)
 
+if(@protobuf_BUILD_PROTOC_BINARIES@)
+
 # The Protobuf Protoc Library
 _protobuf_find_libraries(Protobuf_PROTOC protoc)
 
+endif()
+
 if(UNIX)
   _protobuf_find_threads()
 endif()