Use correct types in string-set.h

My earlier patch to introduce string-set.h used the wrong type in the
hash functions.  This patch fixes the error.

gdbsupport/string-set.h

diff --git a/gdbsupport/string-set.h b/gdbsupport/string-set.h
index c250745..c9078fc 100644
--- a/gdbsupport/string-set.h
+++ b/gdbsupport/string-set.h
@@ -109,12 +109,12 @@ class string_set
     using is_transparent = void;
     using is_avalanching = void;
 
-    bool operator() (const local_string &rhs) const noexcept
+    uint64_t operator() (const local_string &rhs) const noexcept
     {
       return (*this) (rhs.as_view ());
     }
 
-    bool operator() (std::string_view rhs) const noexcept
+    uint64_t operator() (std::string_view rhs) const noexcept
     {
       return ankerl::unordered_dense::hash<std::string_view> () (rhs);
     }