Use C99 syntax for font drivers

Use C99 syntax for font drivers

Problem reported by Daniel Colascione in:
http://lists.gnu.org/archive/html/emacs-devel/2016-12/msg00515.html
* src/ftcrfont.c (ftcrfont_driver):
* src/ftfont.c (ftfont_driver):
* src/ftxfont.c (ftxfont_driver):
* src/macfont.m (macfont_driver):
* src/nsfont.m (nsfont_driver):
* src/xfont.c (xfont_driver):
* src/xftfont.c (xftfont_driver):
Use C99 syntax, not the old GNU C syntax.