Add annotations to create gnss-base.h

This is required to make sure that the constants in gps.h does not
ever go out of sync with the enums created out of them in the HAL
interface definition.

Test: mm

Change-Id: I103cf46d198e9f5de9e0cfd64f2795f0094ecb39
diff --git a/gnss/1.0/IAGnss.hal b/gnss/1.0/IAGnss.hal
index 2cce519..b8f5746 100644
--- a/gnss/1.0/IAGnss.hal
+++ b/gnss/1.0/IAGnss.hal
@@ -22,6 +22,7 @@
  * Extended interface for AGNSS support.
  */
 interface IAGnss {
+    @export(name="", value_prefix="APN_IP_")
     enum ApnIpType : uint8_t {
         INVALID  = 0,
         IPV4     = 1,