selftests: drivers/dma-buf: Fix implicit declaration warns

udmabuf has the following implicit declaration warns:

udmabuf.c:30:10: warning: implicit declaration of function 'open';
udmabuf.c:42:8: warning: implicit declaration of function 'fcntl'

These are caused due to not including fcntl.h and including just
linux/fcntl.h. Fix it to include fcntl.h which will bring in the
linux/fcntl.h. In addition, define __EXPORTED_HEADERS__ to bring in
F_ADD_SEALS and F_SEAL_SHRINK defines and fix the following error
that show up when just fcntl.h is included.

udmabuf.c:45:21: error: 'F_ADD_SEALS' undeclared
   45 |  ret = fcntl(memfd, F_ADD_SEALS, F_SEAL_SHRINK);
      |                     ^~~~~~~~~~~
udmabuf.c:45:34: error: 'F_SEAL_SHRINK' undeclared
   45 |  ret = fcntl(memfd, F_ADD_SEALS, F_SEAL_SHRINK);
      |                                  ^~~~~~~~~~~~~

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>