Don't try to exhaustively anticipate all generated header names

When JNI headers for a given class, each nested class will end up with
its own additional header.  But I don't want to try to parse nested
class details out of the Java source files just so we can determine
exactly which headers will be created.  Instead, simply treat the
entire header destination directory as the output of this task.

com.ibm.wala.cast.test/build.gradle

@@ -32,7 +32,7 @@ task generateJniHeaders(type: Exec, dependsOn: [testClasses, ':com.ibm.wala.cast
 		headerDir = "$buildDir/include"
 	}
 	inputs.files files(qualifiedClasses.collect { "$buildDir/${it.replace('.', '/')}.class" })
-	outputs.files files(qualifiedClasses.collect { "$headerDir/${it.replace('.', '_')}.h" })
+	outputs.dir headerDir
 	outputs.cacheIf { true }
 
 	// javah command to generate headers