`#include` `<cmath>` instead of `<math.h>`.
`#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 <d2756efe887c28699f4587ef278926ee3b3bfae5@google.com>
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2