kselftests: dmabuf-heaps: Fix Makefile's inclusion of the kernel's usr/include dir

Copied in from somewhere else, the makefile was including
the kerne's usr/include dir, which caused the asm/ioctl.h file
to be used.

Unfortunately, that file has different values for _IOC_SIZEBITS
and _IOC_WRITE than include/uapi/asm-generic/ioctl.h which then
causes the _IOCW macros to give the wrong ioctl numbers,
specifically for DMA_BUF_IOCTL_SYNC.

This patch simply removes the extra include from the Makefile

Cc: Shuah Khan <shuah@kernel.org>
Cc: Brian Starkey <brian.starkey@arm.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Laura Abbott <labbott@kernel.org>
Cc: Hridya Valsaraju <hridya@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sandeep Patil <sspatil@google.com>
Cc: Daniel Mentz <danielmentz@google.com>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kselftest@vger.kernel.org
Fixes: a8779927 ("kselftests: Add dma-heap test")
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>