genksyms: Use same type in loop comparison

The ARRAY_SIZE macro in scripts/genksyms/genksyms.c returns a value of
type size_t. That value is being compared to a variable of type int in
a loop in read_node(). Change the int variable to size_t type as well,
so we don't do signed vs unsigned type comparisons with all the
potential promotion/sign extension trouble that can cause (also
silences compiler warnings at high levels of warnings).

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>