package/libv4l: no libv4lconvert helper support if no fork()

package/libv4l: no libv4lconvert helper support if no fork()

Build libv4lconvert helper support only when fork()
is available.

Fixes [1]:
    CXXLD    v4l2-compliance
   .../build/libv4l-1.12.5/lib/libv4lconvert/.libs/libv4lconvert.so: undefined reference to `fork'
  collect2: error: ld returned 1 exit status

[1] http://autobuild.buildroot.net/results/7e8/7e8fbd99a8c091d7bbeedd16066297682bbe29fe

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>