fix build error

Author: Alami-Amine

build/config/compiler/BUILD.gn

@@ -380,6 +380,9 @@ config("runtime_default") {
   # the clang_sysroot folder is populated from CIPD  (fuchsia/third_party/sysroot/linux) which does not include the libatomic library and makes the FuzzTest linking fail
   # FYI, using the sysroot flag is only activated when pw_toolchain_OSS_FUZZ_ENABLED=false
   if (is_pw_fuzz == false) {
+    if (!defined(libs)) {
+      libs = []
+    }
     libs += [ "atomic" ]
   }