not tested your code.so just check if this works.

var x refcursor
exec GetEmployeeDtl(20,:x);

and remove the quotes around 20 it's not needed.

ERROR at line 1:
ORA-00900: invalid SQL statement
ORA-06512: at "GETEMPLOYEEDTL", line 14
ORA-06512: at line 4

CREATE OR REPLACE PROCEDURE GetEmployeeDtl (p_deptno IN
emp.deptno%TYPE,
p_emp OUT SYS_REFCURSOR) AS
v_var varchar2(4000);
BEGIN
--v_var:='OPEN p_emp FOR';
v_var:= ' SELECT ename,';
v_var:= v_var || ' empno,';
v_var:= v_var || ' deptno';
v_var:= v_var || ' FROM emp';
v_var:= v_var || ' WHERE deptno = ' ||p_deptno;
v_var:= v_var || ' ORDER BY ename';

OPEN p_emp FOR v_var;

END GetEmployeeDtl;

CREATE OR REPLACE PROCEDURE GetEmployeeDtl (p_deptno IN
emp.deptno%TYPE,
p_emp OUT SYS_REFCURSOR) AS
BEGIN
OPEN p_emp FOR 
  SELECT ename
        ,empno
        ,deptno 
  FROM   emp 
  WHERE  deptno = 40 
  ORDER BY ename;

END GetEmployeeDtl;

  SQL> CREATE OR REPLACE PROCEDURE GetEmployeeDtl (p_deptno IN
  2  emp.deptno%TYPE,
  3  p_emp OUT SYS_REFCURSOR) AS
  4  v_var varchar2(4000);
  5  BEGIN
  6  v_var:='OPEN p_emp FOR';
  7  v_var:= ' SELECT ename,';
  8  v_var:= v_var || ' empno,';
  9  v_var:= v_var || ' deptno';
 10  v_var:= v_var || ' FROM emp';
 11  v_var:= v_var || ' WHERE deptno = ' ||p_deptno;
 12  v_var:= v_var || ' ORDER BY ename';
 13  execute immediate  v_var ;
 14* END GetEmployeeDtl;
 15  /

Procedure created.

SQL> var x refcursor
SQL> exec GetEmployeeDtl(20,:x)

PL/SQL procedure successfully completed.

CREATE OR REPLACE PROCEDURE GetEmployeeDtl (p_deptno IN
emp.deptno%TYPE,
p_emp OUT SYS_REFCURSOR) AS
v_var varchar2(4000);
BEGIN
--v_var:='OPEN p_emp FOR';
v_var:= ' SELECT ename,';
v_var:= v_var || ' empno,';
v_var:= v_var || ' deptno';
v_var:= v_var || ' FROM emp';
v_var:= v_var || ' WHERE deptno = ' ||p_deptno;
v_var:= v_var || ' ORDER BY ename';

OPEN p_emp FOR v_var;

END GetEmployeeDtl;

1 CREATE OR REPLACE PROCEDURE GetEmployeeDtl (p_deptno IN
2 emp.deptno%TYPE,
3 p_emp OUT SYS_REFCURSOR) AS
4 v_var varchar2(4000);
5 BEGIN
6 v_var:='OPEN p_emp FOR';
7 v_var:= ' SELECT ename,';
8 v_var:= v_var || ' empno,';
9 v_var:= v_var || ' deptno';
0 v_var:= v_var || ' FROM emp';
1 v_var:= v_var || ' WHERE deptno = ' ||p_deptno;
2 v_var:= v_var || ' ORDER BY ename';
3 execute immediate v_var ;
4* END GetEmployeeDtl;
5 /

SQL>  variable x refcursor;
SQL> EXEC Getemployeedtl(20,:x)
SQL>  print x;
ERROR:
ORA-24338: statement handle not executed
SP2-0625: Error printing variable "x"

execute immediate v_var ;