Use realfilename for FcOpen in _FcConfigParse

realfilename is the file name after sysroot adjustments.  It should be used
instead of filename in the call to FcOpen() which forwards the name directly to
open().

Though I don't explicitly request a sysroot, I was getting error messages saying
"failed reading config file".  This CL fixes the error spam.

src/fcxml.c

@@ -3443,7 +3443,7 @@ _FcConfigParse (FcConfig	*config,
 
     FcStrBufInit (&sbuf, NULL, 0);
 
-    fd = FcOpen ((char *) filename, O_RDONLY);
+    fd = FcOpen ((char *) realfilename, O_RDONLY);
     if (fd == -1)
 	goto bail1;