fix ck_assert_ptr_* causing const compilation warnings

fix ck_assert_ptr_* causing const compilation warnings

A test sequence such as:

const struct my_vtable expected_element;
struct my_vtable* test_element;
ck_assert_ptr_eq(test_element, &expected_element);

Compiled using gcc 4.7.2 with -Wall gives the following warning.
myfile.c:70:2: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]

Adding a const qualifier to the variables in the _ck_assert_ptr macro,
as done for _ck_assert_str makes the problem go away.

Issue #91, patch submitted by user lordlod24.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@861 64e312b2-a51f-0410-8e61-82d0ca0eb02a