tests: kernel/smp: whitelist qemu_x86_64 for testing

Add qemu_x86_64 to the platform whitelist so that this will actually
be built and tested with sanitycheck.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

tests/kernel/smp/testcase.yaml

@@ -1,3 +1,3 @@
 tests:
   kernel.multiprocessing:
-    platform_whitelist: esp32
+    platform_whitelist: esp32 qemu_x86_64