Bug 1666030 - Fix floor missing on some builds r=dmajor

Depends on D91004

Differential Revision: https://phabricator.services.mozilla.com/D91284

mozglue/misc/PreXULSkeletonUI.cpp

@@ -7,6 +7,7 @@
 #include "PreXULSkeletonUI.h"
 
 #include <algorithm>
+#include <math.h>
 
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"