Add missing std prefix causing compilation warnings

Author: xzyfer

src/eval.cpp

@@ -31,7 +31,7 @@ namespace Sass {
   inline double sub(double x, double y) { return x - y; }
   inline double mul(double x, double y) { return x * y; }
   inline double div(double x, double y) { return x / y; } // x/0 checked by caller
-  inline double mod(double x, double y) { return abs(fmod(x, y)); } // x/0 checked by caller
+  inline double mod(double x, double y) { return std::abs(std::fmod(x, y)); } // x/0 checked by caller
   typedef double (*bop)(double, double);
   bop ops[Sass_OP::NUM_OPS] = {
     0, 0, // and, or