When the kernel is compiled with allmodconfig, the following error is
reported:

In file included from drivers/scsi/ufs/ufs-mediatek-trace.h:36:0,
                 from drivers/scsi/ufs/ufs-mediatek.c:28:
./include/trace/define_trace.h:95:42: fatal error: ./ufs-mediatek-trace.h: No such file or directory
 #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)

The comment in include/trace/define_trace.h specifies that:
TRACE_INCLUDE_PATH: Note, the path is relative to define_trace.h, not the
file including it. Full path names for out of tree modules must be used.

So without "CFLAGS_ufs-mediatek.o := -I$(src)", the current directory "."
is "include/trace/", the relative path of ufs-mediatek-trace.h is
"../../drivers/scsi/ufs/".

Link: https://lore.kernel.org/r/20201209063144.1840-2-thunder.leizhen@huawei.com
Fixes: ca1bb061 ("scsi: ufs-mediatek: Introduce event_notify implementation")
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>