create-diff-object: fix symtab sh_info field

This fixes the weird ld errors we've been seeing lately.

According to the "ELF-64 Object File Format" spec, the symtab sh_info
field should contain "Index of first non-local symbol (i.e., number of
local symbols)".
Merge pull request #206 from jpoimboe/symtab-sh-info

create-diff-object: fix symtab sh_info field