Use gdb::unique_ptr in elf_read_minimal_symbols

Use gdb::unique_ptr in elf_read_minimal_symbols

This changes elf_read_minimal_symbols to use gdb::unique_ptr rather
than an explicit allocation.  This removes a cleanup.

2016-10-21  Tom Tromey  <tom@tromey.com>

	* elfread.c (elf_read_minimal_symbols): Use gdb::unique_ptr.