scripts: modpost: fix compilation warning

The scripts/mod/modpost.c triggers the following warning:

scripts/mod/modpost.c: In function ‘remove_dot’:
scripts/mod/modpost.c:1710:10: warning: ignoring return value of ‘strtoul’, declared with attribute warn_unused_result [-Wunused-result]

The remove_dot function that calls strtoul does not care about the
numeric value of the string that is parsed but only looks for the
end of the numeric sequence.  As such, it's equivalent to just skip
over all digits.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>