objcopy.c: Fix bfd_copy_private_symbol_data on 32-bit hosts

Use long with bfd_copy_private_symbol_data to fix

.../binutils/objcopy.c: In
function ‘copy_object’:
.../binutils/objcopy.c:3383:17: error: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘long int’ [-Werror=sign-compare]
 3383 |   for (i = 0; i < symcount; i++)
      |                 ^

on 32-bit hosts.

	PR binutils/14493
	* objcopy.c (copy_object): Use long with
	bfd_copy_private_symbol_data.