Fix crash due to invalid timespec given to pselect

If blinktimeout is set to a value greater than 1000, pselect will
receive a timeout argument with tv_nsec greater than 1E9 (1 sec), and
fail, making st crash. This patch just ensures that the timespec
structure is correctly filled with a value properly decomposed between
tv_sec and tv_nsec.

Reported by JasonWoof on IRC. Thanks!
my config
solarized dark color scheme