Don't build panic.c in two different targets.

Change-Id: I9913017b15876009bd90c65de122d14932617863
diff --git a/src/BUILD.gn b/src/BUILD.gn
index c9ecd1a..66c9211 100644
--- a/src/BUILD.gn
+++ b/src/BUILD.gn
@@ -56,7 +56,6 @@
     "api.c",
     "cpu.c",
     "manifest.c",
-    "panic.c",
     "spci_memory.c",
     "vcpu.c",
   ]
@@ -68,6 +67,7 @@
     ":fdt_handler",
     ":memiter",
     ":mm",
+    ":panic",
     ":std",
     ":vm",
     "//src/arch/${plat_arch}/hypervisor",