First reference beignet's CL header to build
First reference beignet's CL header to build

This is to fix build error when new intel extension is added
into beignet. As current cmake rule will use old system installed
CL headers instead of beignet ones, which leads to compile failure
as new extension definition won't be found. So this trys to simply
always prefer to use beignet's CL header in build/install.

Signed-off-by: Zhenyu Wang <2be145655f058b1e18fb2a9466bd2b1b12e2cd7e@linux.intel.com>
Reviewed-by: "Yang, Rong R" <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet