examples/libcoap: fix compile error in example.
examples/libcoap: fix compile error in example.

This commit is to fix below compilation error on client example.

CC:  libcoap-client.c
libcoap-client.c:787:47: error: '_token_data' is static but used
            in inline function 'cmdline_token' which is not static [-Werror]
  strncpy((char *)the_token.s, arg, min(sizeof(_token_data), strlen(arg)));
                                               ^
libcoap-client.c:82:32: note: in definition of macro 'min'
 #define min(a,b) ((a) < (b) ? (a) : (b))
                                ^
ibcoap-client.c:787:47: error: '_token_data' is static but used
            in inline function 'cmdline_token' which is not static [-Werror]
  strncpy((char *)the_token.s, arg, min(sizeof(_token_data), strlen(arg)));
                                               ^
libcoap-client.c:82:20: note: in definition of macro 'min'
 #define min(a,b) ((a) < (b) ? (a) : (b))
                    ^
Signed-off-by: Junyeon LEE <junyeon2.lee@samsung.com>

Change-Id: I820e3c872ca100b9e34027e1d28e4b4f42c78d11

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