Fixed unused function error that caused headless builds to fail.

The function is anonymous::SelectedLinuxBackendToString() and it is
called from KeyStorageLinux::CreateService from under a bunch of
#ifdef's

Change-Id: Ia087cf602b0fc09c76e2f3c77322a66680ed6d30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900114
Commit-Queue: Peter Kvitek <kvitekp@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712719}

components/os_crypt/key_storage_linux.cc

@@ -34,6 +34,8 @@ const char KeyStorageLinux::kFolderName[] = "Chromium Keys";
 const char KeyStorageLinux::kKey[] = "Chromium Safe Storage";
 #endif
 
+#if defined(USE_LIBSECRET) || defined(USE_KEYRING) || defined(USE_KWALLET)
+
 namespace {
 
 const char* SelectedLinuxBackendToString(
@@ -60,6 +62,9 @@ const char* SelectedLinuxBackendToString(
 
 }  // namespace
 
+#endif  // defined(USE_LIBSECRET) || defined(USE_KEYRING) ||
+        // defined(USE_KWALLET)
+
 // static
 std::unique_ptr<KeyStorageLinux> KeyStorageLinux::CreateService(
     const os_crypt::Config& config) {