[fuchsia] Add base/log_severity.cc to BUILD.gn

This is referenced in base/log_severity.h but was not built.

Change-Id: I3a33fc8ffbfd45a8ca8445992a397fd839a58db4
diff --git a/absl/base/BUILD.gn b/absl/base/BUILD.gn
index 48b1b4c..7a6d70e 100644
--- a/absl/base/BUILD.gn
+++ b/absl/base/BUILD.gn
@@ -50,6 +50,7 @@
     "internal/sysinfo.cc",
     "internal/thread_identity.cc",
     "internal/unscaledcycleclock.cc",
+    "log_severity.cc",
   ]
   public = [
     "call_once.h",