exfat: fix build on x86 with 4.19 kernel

This should fix:

balloc.c:325:7: error: implicit declaration of function
'fatal_signal_pending'; did you mean 'mod_timer_pending'?
[-Werror=implicit-function-declaration]
   if (fatal_signal_pending(current)) {
       ^~~~~~~~~~~~~~~~~~~~
       mod_timer_pending
cc1: some warnings being treated as errors

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>