epee: include math header for upcoming Boost 1.89

When building on Ubuntu 22.04 with GCC 11.4.0 and Boost 1.89.0.beta1,
the build failed with:
```
error: ‘floor’ was not declared in this scope
     41 |       num_char = (int)floor((float)num_char / (float)radix);
        |                       ^~~~~
```

Signed-off-by: Michael Cho <michael@michaelcho.dev>

contrib/epee/src/abstract_http_client.cpp

@@ -1,3 +1,4 @@
+#include <math.h>
 #include "net/abstract_http_client.h"
 #include "net/http_base.h"
 #include "net/net_parse_helpers.h"