fix: separate BigInteger declarations to satisfy Checkstyle

Author: polasisubash

src/main/java/com/thealgorithms/ciphers/ElGamalCipher.java

@@ -4,7 +4,11 @@
 import java.security.SecureRandom;
 
 public class ElGamalCipher {
-    private BigInteger p, g, x, y;
+    private BigInteger p;
+    private BigInteger g;
+    private BigInteger x;
+    private BigInteger y;
+
     private SecureRandom random = new SecureRandom();
 
     // Key generation