Fix extra compiler warnings

Fix extra compiler warnings

Fixes following -Wextra compiler warnings:

 uclient.c:195:16: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
         for (i = 0; i < ARRAY_SIZE(backends); i++) {
                     ~ ^ ~~~~~~~~~~~~~~~~~~~~

 uclient-http.c:619:2: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
         blobmsg_for_each_attr(cur, uh->headers.head, rem)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 uclient-http.c:619:2: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
         blobmsg_for_each_attr(cur, uh->headers.head, rem)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 uclient-http.c:993:16: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
         for (i = 0; i < ARRAY_SIZE(request_types); i++) {
                     ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~

 uclient.c:195:16: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
         for (i = 0; i < ARRAY_SIZE(backends); i++) {
                     ~ ^ ~~~~~~~~~~~~~~~~~~~~

 uclient-http.c:619:2: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
         blobmsg_for_each_attr(cur, uh->headers.head, rem)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 uclient-http.c:619:2: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
         blobmsg_for_each_attr(cur, uh->headers.head, rem)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 uclient-http.c:993:16: error: comparison of integers of different signs: 'int' and 'unsigned long' [-Werror,-Wsign-compare]
         for (i = 0; i < ARRAY_SIZE(request_types); i++) {
                     ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~

 uclient-fetch.c:551:67: error: missing field 'flag' initializer [-Werror,-Wmissing-field-initializers]
         [L_NO_CHECK_CERTIFICATE] = { "no-check-certificate", no_argument },

Signed-off-by: Petr Štetiar <ynezz@true.cz>