fix -Wsign-compare warnings (#369)

Fix repeated errors like this:

    log_format.hh:1138:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (len < jfe.jfe_min_width) {

Fixes #369.

https://github.com/tstack/lnav/issues/369

src/log_format.hh

@@ -1018,8 +1018,8 @@ public:
         json_log_field jfe_type;
         intern_string_t jfe_value;
         std::string jfe_default_value;
-        long long jfe_min_width;
-        long long jfe_max_width;
+        unsigned long long jfe_min_width;
+        unsigned long long jfe_max_width;
         align_t jfe_align;
         overflow_t jfe_overflow;
         std::string jfe_ts_format;

src/yajlpp.hh

@@ -273,7 +273,7 @@ struct json_path_handler : public json_path_handler_base {
         return *this;
     };
 
-    json_path_handler &for_field(long long *field) {
+    json_path_handler &for_field(unsigned long long *field) {
         this->add_cb(yajlpp_static_number);
         this->jph_simple_offset = field;
         this->jph_validator = yajlpp_validator_for_int;