lib/zoap: Fix warning about signedness conversion

'query->value' is a 'const uint8_t *' so it should be casted to a
char (signed) array before it is used in places where a 'char *'
is expected, strncmp() is an example.

Jira: ZEP-1810

Change-Id: I94cf780a40ad5fed29607d2302dc7a10387bb86f
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>