cmake: remove spurious parenthese just added to xt-ld/linker_flags

Fixes commit 9a9e252d ("cmake: linker: move toolchain_ld_<base|cpp>
to linker flag property")

Fixes failure:

```
CMake Error at zephyr/cmake/linker/xt-ld/linker_flags.cmake:32:
  Parse error.  Expected a command name, got right paren with text ")".

Call Stack (most recent call first):
  zephyr/cmake/target_toolchain_flags.cmake:42 (include)
  zephyr/cmake/modules/kernel.cmake:148 (include)
  zephyr/cmake/modules/zephyr_default.cmake:142 (include)
  zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:66 (include)
  zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:97 (include_boi
```

Signed-off-by: Marc Herbert <marc.herbert@intel.com>