exfat: further fix 'fatal_signal_pending' undeclared

Make sure to include the header file that defines this function.

Fixes: 19b70873 ("exfat: fix build on x86 with 4.19 kernel")
Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>