libv4l: decode-tm6000 requires libv4l2util

The decode-tm6000 utility cannot build without the libv4l2util. If
this library is not available, the build breaks with:

decode_tm6000.o: In function `read_stream':
decode_tm6000.c:(.text+0x220): undefined reference to `v4l2_rcvbuf'
decode_tm6000.o: In function `main':
decode_tm6000.c:(.text+0x37c): undefined reference to `v4l2_open'
decode_tm6000.c:(.text+0x3cc): undefined reference to `v4l2_gettryset_fmt_cap'
decode_tm6000.c:(.text+0x424): undefined reference to `v4l2_getset_freq'
decode_tm6000.c:(.text+0x47c): undefined reference to `v4l2_mmap_bufs'
decode_tm6000.c:(.text+0x4a0): undefined reference to `v4l2_start_streaming'

See

 http://autobuild.buildroot.org/results/207ed74d5e816309ef0dc82ecc8112b51788fdf6/build-end.log



We fix this by adding util/libv4l2util to the list of directories to
build when decode-tm6000 is enabled. The only other user of
libv4l2util is another utility called qv4l2, for which Buildroot has
no Config.in option, so we only handle the case of decode-tm6000 at
the moment.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>