Merge pull request #4580 from shuujii/unify-type-of-line-number-to-uint16_t

Unify type of line number to `uint16_t`

src/backtrace.c

@@ -16,7 +16,7 @@
 #include <mruby/data.h>
 
 struct backtrace_location {
-  int lineno;
+  int32_t lineno;
   mrb_sym method_id;
   const char *filename;
 };