mktime(): Remove redundant assignment of 'result'
Checked with upstream lynx source code and the
second calculation line is the upstream one, too.
(In fact I diffed the whole function to check for changes)
cppcheck reported:
[rockbox/firmware/libc/mktime.c:43] -> [rockbox/firmware/libc/mktime.c:44]: (performance) Variable 'result' is reassigned a value before the old one has been used.
Change-Id: Ia04c5f55da7d86cd74cff4fce675a9c85ddce3e2