community/opencl: split -headers subpackage

currently, some things want to build against the other opencl-icd-loader-dev (which contains a ocl_icd.h that this doesn't), but that package does not contain the rest of the opencl headers- and it cannot pull them in from this package (i.e. apk add opencl-headers opencl-icd-loader-dev, which pulls in opencl-dev) because both dev packages have a libOpenCL.so symlink to their respective loader implementation (which are both libOpenCL.so.1). this makes them incompatible- but splitting the headers like this allows that addition to work and either loader to be used, while not affecting this opencl package (as the -dev will pull in the same files)