v4l-utils: fix make install problems with ln -s

v4l-utils: fix make install problems with ln -s

When re-installing the linking of the some libraries fail with a 'File exists'
error:

make  install-exec-hook
make[4]: Entering directory `/home/hve/work/src/v4l/v4l-utils/lib/libv4l2'
/bin/mkdir -p //usr/local/lib
(cd //usr/local/lib && ln -s libv4l/v4l2convert.so v4l2convert.so)
ln: failed to create symbolic link 'v4l2convert.so': File exists
make[4]: *** [install-exec-hook] Error 1
make[4]: Leaving directory `/home/hve/work/src/v4l/v4l-utils/lib/libv4l2'
make[3]: *** [install-exec-am] Error 2
make[3]: Leaving directory `/home/hve/work/src/v4l/v4l-utils/lib/libv4l2'
make[2]: *** [install-am] Error 2
make[2]: Leaving directory `/home/hve/work/src/v4l/v4l-utils/lib/libv4l2'
make[1]: *** [install-recursive] Error 1
make[1]: Leaving directory `/home/hve/work/src/v4l/v4l-utils/lib'
make: *** [install-recursive] Error 1

Fix this by removing the old link first.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>