`#include` `<cmath>` instead of `<math.h>`.

With the latter, AppleClang seems not to have `log()` in the `std`
namespace...

Change-Id: I10d8c8210af55d409f00c08e6ab864cea99cd0d4
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60070
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/re2/prefilter_tree.cc b/re2/prefilter_tree.cc
index af50b31..f72b72f 100644
--- a/re2/prefilter_tree.cc
+++ b/re2/prefilter_tree.cc
@@ -4,9 +4,9 @@
 
 #include "re2/prefilter_tree.h"
 
-#include <math.h>
 #include <stddef.h>
 #include <algorithm>
+#include <cmath>
 #include <map>
 #include <memory>
 #include <string>