Prevent GCC 8 from reporting error in ClassLoader::file_name_for_class_name()

src/hotspot/share/classfile/classLoader.cpp: In static member function ‘static const char* ClassLoader::file_name_for_class_name(const char*, int)’:
src/hotspot/share/classfile/classLoader.cpp:1233:53: error: argument to ‘sizeof’ in ‘char* strncpy(char*, const char*, size_t)’ call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
   strncpy(&file_name[class_name_len], class_suffix, sizeof(class_suffix));
                                                     ^~~~~~~~~~~~~~~~~~~~

The code itself looks correct and it seems having variable for suffix length should be enough.