[SYCL][CMake] Build sycl-headers before bfloat16 objects (intel#8745)

bfloat16_wrapper.cpp requires SYCL headers, so ensure that they've been
staged to the build directory before compiling.

libdevice/cmake/modules/SYCLLibdevice.cmake

@@ -111,7 +111,7 @@ set(complex_obj_deps device_complex.h device.h sycl-compiler)
 set(cmath_obj_deps device_math.h device.h sycl-compiler)
 set(imf_obj_deps device_imf.hpp imf_half.hpp imf_bf16.hpp device.h sycl-compiler)
 set(itt_obj_deps device_itt.h spirv_vars.h device.h sycl-compiler)
-set(bfloat16_obj_deps sycl-compiler)
+set(bfloat16_obj_deps sycl-headers sycl-compiler)
 
 add_devicelib_obj(libsycl-itt-stubs SRC itt_stubs.cpp DEP ${itt_obj_deps})
 add_devicelib_obj(libsycl-itt-compiler-wrappers SRC itt_compiler_wrappers.cpp DEP ${itt_obj_deps})