Include math.h for corelib.c

This is the downside of the forced amalgamated build. Strange build
constraints.

src/core/util.h

@@ -33,6 +33,7 @@
 #include <errno.h>
 #include <stddef.h>
 #include <stdbool.h>
+#include <math.h>
 
 #ifdef JANET_EV
 #ifndef JANET_WINDOWS