mp4v2: fix help2man-related build failure

mp4v2: fix help2man-related build failure

Fixes:
http://autobuild.buildroot.net/results/62a/62af25aace6b17492135e3567f3fa14cc05fef75/
http://autobuild.buildroot.net/results/043/043e23a8a1575f249d4654e2ce5f4a24e29d080a/
http://autobuild.buildroot.net/results/c6f/c6f1f7eef34025f736e35b5714029f5190bcb648/
http://autobuild.buildroot.net/results/bc4/bc4beb415491fdfb65639c0d51ae75a1789c68dd/

And many others.

Help2man expects to be able to run utilities on the build machine to grab
--help output which doesn't work when cross compiling, so disable it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>