core: don't explicitly use std:: prefix when the platform may not

recognise it.

src/matrix.h

@@ -113,7 +113,8 @@ namespace lol
     \
     inline float len() const \
     { \
-        return std::sqrtf((float)sqlen()); \
+        using namespace std; \
+        return sqrtf((float)sqlen()); \
     }
 
 template <typename T> struct Vec2;

src/numeric.h

@@ -26,7 +26,8 @@ namespace lol
 /* Random float value */
 static inline float RandF()
 {
-    return (float)std::rand() / RAND_MAX;
+    using namespace std;
+    return (float)rand() / RAND_MAX;
 }
 
 static inline float RandF(float val)