Allow signals to hal_graphics_allocator_server

This is needed to dump ANR traces for the process.

Bug: 111604912
Test: adb shell am hang
Change-Id: Icadc2de95a12818fe623271d1afb955843ccddbf

private/system_server.te

@@ -260,6 +260,7 @@ allow system_server {
   hal_audio_server
   hal_bluetooth_server
   hal_camera_server
+  hal_graphics_allocator_server
   hal_graphics_composer_server
   hal_omx_server
   hal_sensors_server