Address `-Wunused-parameter' warnings.

Change-Id: I2b1f21d6b5b27d09521a1fb57d7694d14a7a3446
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57890
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/re2/re2.h b/re2/re2.h
index 2197b3f..6e93401 100644
--- a/re2/re2.h
+++ b/re2/re2.h
@@ -819,7 +819,7 @@
  public:
   Arg() : Arg(nullptr) {}
   Arg(std::nullptr_t ptr)
-      : arg_(ptr), parser_([](const char* str, size_t n, void* dest) -> bool {
+      : arg_(ptr), parser_([](const char* /*str*/, size_t /*n*/, void* /*dest*/) -> bool {
           return true;
         }) {}