elfio_note: fix MSVC compiler warning

Fix the following MSVC compiler warning:

elfio/elfio_note.hpp(77): warning C4267: 'initializing' : conversion from 'size_t' to 'ELFIO::Elf_Word', possible loss of data

by changing the type of max_name_sizei to Elf_Xword, as
note_secton->get_size() returns Elf_Xword and note_start_positions also
contains members of type Elf_Xword.