LibC: Add `labs()`

We defined it in `stdlib.h` but forgot to implement it.

Userland/Libraries/LibC/stdlib.cpp

@@ -725,6 +725,11 @@ int abs(int i)
     return i < 0 ? -i : i;
 }
 
+long int labs(long int i)
+{
+    return i < 0 ? -i : i;
+}
+
 long long int llabs(long long int i)
 {
     return i < 0 ? -i : i;