`#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>
1 file changed