FcName{,Get}Constant: constify string input

FcName{,Get}Constant: constify string input

These funcs don't modify the incoming string, so add const markings.
This is the "right thing", shouldn't change the ABI, and fixes some
gcc warnings:

fccfg.c: In function 'FcConfigEvaluate':
fccfg.c:916:2: warning: passing argument 1 of 'IA__FcNameConstant'
	discards 'const' qualifier from pointer target type [enabled by default]
fcalias.h:253:34: note: expected 'FcChar8 *' but
	argument is of type 'const FcChar8 *'

fcxml.c: In function 'FcTypecheckExpr':
fcxml.c:604:2: warning: passing argument 1 of 'IA__FcNameGetConstant'
	discards 'const' qualifier from pointer target type [enabled by default]
fcalias.h:251:37: note: expected 'FcChar8 *' but
	argument is of type 'const FcChar8 *'

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