Fixed APP_PLATFORM setting

Author: inclement

pythonforandroid/archs.py

@@ -133,7 +133,7 @@ def get_env(self, with_flags_in_cc=True):
         env['PATH'] = environ['PATH']
 
         env['ARCH'] = self.arch
-        env['NDK_API'] = str(self.ctx.ndk_api)
+        env['NDK_API'] = 'android-{}'.format(str(self.ctx.ndk_api))
 
         if self.ctx.python_recipe and self.ctx.python_recipe.from_crystax:
             env['CRYSTAX_PYTHON_VERSION'] = self.ctx.python_recipe.version