Fix compiling issue with GCC-8.x in string test functions

Same pointer is passed to two or more restrict-qualified parameters of a function.

vpp/src/plugins/unittest/string_test.c: In function ‘test_strcpy_s’:
vpp/src/plugins/unittest/string_test.c:562:19: error: passing argument 1 to restrict-qualified parameter aliases with argument 3 [-Werror=restrict]
err = strcpy_s (dst, s1size, dst);
^~~ ~~~

Change-Id: Ica06b457bbcbf2d552eec380976c37f9fd447b1c
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Sirshak Das <sirdas@arm.com>