Add a path separator to OCL_ICD_VENDORS

Add a path separator to OCL_ICD_VENDORS

When using the `OCL_ICD_VENDORS` environment variable a trailing `/`
path separator must be provided by the user, this is surprising.

This patch updates the `sprintf` call to include a `/` when constructing
the `.icd` file path. It also removes the trailing `/` from instances of
the `ICD_VENDOR_PATH` to canonicalised path strings. Existing scripts
which specify a trailing `/` when setting `OCL_ICD_VENDORS` will
continue working as expected since duplicate path separators `//`, while
not canonical, work as if a single path separator is specified.