Fix ant build on Windows.

android/custom_rules.xml

@@ -1,14 +1,17 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project name="PPSSPP" default="help">
+    <condition property="ndkbuild_exe" value="ndk-build.cmd" else="ndk-build">
+        <os family="windows" />
+    </condition>
     <property name="ndkbuildopt" value=""/>
     <target name="-pre-build">
-        <exec executable="${ndk.dir}/ndk-build" failonerror="true">
+        <exec executable="${ndk.dir}/${ndkbuild_exe}" failonerror="true">
             <arg line="-j4 ${ndkbuildopt}"/>
             <env key="NDK_MODULE_PATH" path="../ext:../ext/native/ext"/>
         </exec>
     </target>
     <target name="clean" depends="android_rules.clean">
-        <exec executable="${ndk.dir}/ndk-build" failonerror="true">
+        <exec executable="${ndk.dir}/${ndkbuild_exe}" failonerror="true">
             <arg value="clean"/>
             <env key="NDK_MODULE_PATH" path="../ext:../ext/native/ext"/>
         </exec>