LibC, LibM: Add functions needed to compile python3

Author: https://github.com/lnzero1dev
Commit: e7affa24dc
Pull-request: https://github.com/SerenityOS/serenity/pull/763
Reviewed-by: https://github.com/awesomekling ✅
Reviewed-by: https://github.com/bugaevc

Libraries/LibM/math.cpp

@@ -301,4 +301,10 @@ float ceilf(float value)
     }
     return as_int + 1;
 }
+
+double modf(double x, double* intpart)
+{
+    *intpart = (double)((int)(x));
+    return x - (int)x;
+}
 }