build: Pin xxhash to 0.7.0 for gcc-15 compatibility

Otherwise gem install fails on newer Mac OS versions

Author: kspurgin

Gemfile.lock

@@ -23,7 +23,7 @@ PATH
       marc
       measured
       thor
-      xxhash
+      xxhash (= 0.7.0)
       zeitwerk
 
 GEM
@@ -202,4 +202,4 @@ DEPENDENCIES
   yardspec (~> 0.2.0)
 
 BUNDLED WITH
-   2.3.9
+   2.7.1

kiba-extend.gemspec

@@ -51,6 +51,6 @@ Gem::Specification.new do |spec|
   spec.add_dependency "measured"
   spec.add_dependency "thor"
   # @todo can replace with stdlib?
-  spec.add_dependency "xxhash"
+  spec.add_dependency "xxhash", "0.7.0"
   spec.add_dependency "zeitwerk"
 end