fc-{list,match}: constify format string

fc-{list,match}: constify format string

We don't free this string anywhere, so mark it const to avoid gcc warnings
and possible bugs in the future (if people did try freeing it).

fc-list.c: In function 'main':
fc-list.c:161:16: warning: pointer targets in assignment
	differ in signedness [-Wpointer-sign]

fc-match.c: In function 'main':
fc-match.c:201:13: warning: pointer targets in assignment
	differ in signedness [-Wpointer-sign]
fc-match.c:203:13: warning: pointer targets in assignment
	differ in signedness [-Wpointer-sign]

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