selftests dma: fix compile error for dma_map_benchmark

When building selftests/dma:
$ make -C tools/testing/selftests TARGETS=dma
I hit the following compilation error:

dma_map_benchmark.c:13:10: fatal error: linux/map_benchmark.h: No such file or directory
#include <linux/map_benchmark.h>
^~~~~~~~~~~~~~~~~~~~~~~

dma/Makefile does not include the map_benchmark.h path, so add
more including path, and fix include order in dma_map_benchmark.c

Fixes: 8ddde07a3d28 ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
Signed-off-by: Yu Liao <[e-mail]>
Tested-by: Shuah Khan <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/dma/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 CFLAGS += -I../../../../usr/include/
+CFLAGS += -I../../../../include/
 
 TEST_GEN_PROGS := dma_map_benchmark
 

tools/testing/selftests/dma/dma_map_benchmark.c

···
 #include <unistd.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
-#include <linux/map_benchmark.h>
 #include <linux/types.h>
+#include <linux/map_benchmark.h>
 
 #define NSEC_PER_MSEC	1000000L