Implemented llabs() in stdlib.h/cpp.

Author: https://github.com/holdengreen
Commit: 18a641f3f8
Pull-request: https://github.com/SerenityOS/serenity/pull/6717
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/gmta
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibC/stdlib.cpp

@@ -713,6 +713,11 @@ int abs(int i)
     return i < 0 ? -i : i;
 }
 
+long long int llabs(long long int i)
+{
+    return i < 0 ? -i : i;
+}
+
 long int random()
 {
     return rand();