package/imlib2: fix musl compilation issue

Fixes http://autobuild.buildroot.net/results/a6e/a6e61bfb376946b479085f243601d6e1634adc6d/



musl is strict about including time.h when using time_t.

Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>