fc-cat: fix pointer warning

fc-cat: fix pointer warning

Add a cast to avoid a gcc warning:

fc-cat.c: In function 'cache_print_set':
fc-cat.c:230:2: warning: pointer targets in passing argument 2
	of 'FcPatternFormat' differ in signedness [-Wpointer-sign]
../fontconfig/fontconfig.h:860:1: note:
	expected 'const FcChar8 *' but argument is of type 'char *'

Signed-off-by: Mike Frysinger <vapier@gentoo.org>