gn: Fix error reported by GN linter

Fix error
```
ERROR at //pigweed/targets/docs/BUILD.gn:68:44:
Build argument has no effect.
    pw_boot_cortex_m_LINK_CONFIG_DEFINES = [
                                           ^
The variable "pw_boot_cortex_m_LINK_CONFIG_DEFINES"
was set as a build argument but never appeared in
a declare_args() block in any buildfile.
```

Change-Id: Ibc8cf1ea83ea1084769851e4e596b75b47ca4988
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/253192
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Docs-Not-Needed: Jiacheng Lu <lujc@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Jiacheng Lu <lujc@google.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>

targets/docs/BUILD.gn

@@ -15,6 +15,7 @@
 import("//build_overrides/pigweed.gni")
 
 import("$dir_pigweed/targets/lm3s6965evb_qemu/target_toolchains.gni")
+import("$dir_pw_boot_cortex_m/toolchain.gni")
 import("$dir_pw_docgen/docs.gni")
 import("$dir_pw_log_tokenized/backend.gni")
 import("$dir_pw_protobuf_compiler/proto.gni")