[tlbuild] Minor fix for poppler

[Karl Berry]

    The following patch is needed for poppler in order to make time_t work
    on FreeBSD 11.1:

    --- libs/poppler/poppler-src/poppler/Form.h (revision 46316)
    +++ libs/poppler/poppler-src/poppler/Form.h (working copy)
    ...
    +#include <ctime>

Committed, thanks. Please tell upstream if you haven't already. -k