x264: disable opencl support

We don't have anything providing opencl support in Buildroot, so this is
just dead code.  More importantly, it uses dlopen so it breaks on completely
static configurations.

Fixes:
http://autobuild.buildroot.net/results/51b/51bd1464fe85f3fcdc5beb095cb1cdbfa413ec24/
http://autobuild.buildroot.net/results/694/694032b21c386c9eb50ba76aa32428bcd5cf9214/
http://autobuild.buildroot.net/results/097/0979f12e50d2413c702147408572b01e6bc700f2/
http://autobuild.buildroot.net/results/d9c/d9c46ca92bc349b42673a60f1b66d6df99d26545/



And many more.

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