[virtmagma] correctly define GPU path in BUILD.gn

This change corrects the parent commit (1cfafcad) to pass the GPU path
override macro to anv_device.c when compiling for the linux toolchain.

Test: ran (pending commit) vulkan_unit_tests in biscotti guest
Change-Id: I038e1d35e12e542071c31458da77968ab5105e26
1 file changed