Bug 1408951 - Fix non-unified build in ErrorNames.cpp. r=froydnj

There is no use mozilla namespace statement in this compilation unit, so the
call to GetErrorName() has to be prefixed with the mozilla namespace.

MozReview-Commit-ID: G3caP2v9PFF

xpcom/base/ErrorNames.cpp

@@ -72,7 +72,7 @@ extern "C" {
 void
 Gecko_GetErrorName(nsresult aRv, nsACString& aName)
 {
-  GetErrorName(aRv, aName);
+  mozilla::GetErrorName(aRv, aName);
 }
 
 }