Add dependency on the .git directory to the dokka task

It includes the current HEAD so it needs to be regenerated if the Git
repository changes.

Signed-off-by: Graham <gpe@openrs2.dev>

build.gradle.kts

@@ -212,6 +212,8 @@ fun commitHash(): String {
 }
 
 tasks.dokka {
+    inputs.dir("$rootDir/.git")
+
     outputDirectory = "$buildDir/kdoc"
     outputFormat = "html"