autotools: Set C++ visibility flags on Intel

These flags are set for C sources, but not C++. This causes symbol
visibility leaks from the C++ parts of the Intel compiler.

Fixes: 700bebb9 ("i965: Move the back-end compiler to src/intel/compiler")
Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 85445549)