fpathcof: _PC_PRIO_IO value of -1 means that unsupported

pse52 test error:fpathconf failed, errno = 38 (ENOSYS)

Signed-off-by: tengshuangshuang <tengshuangshuang@xiaomi.com>

libs/libc/unistd/lib_pathconf.c

@@ -136,6 +136,9 @@ long fpathconf(int fildes, int name)
       case _PC_MAX_INPUT:
         return _POSIX_MAX_INPUT;
 
+      case _PC_PRIO_IO:
+        return -1;
+
       default:
 
         /* Assume valid but not implemented for the time being */