Convert off_t to size_t

include/spdlog/details/os.h

@@ -216,11 +216,11 @@ inline size_t filesize(FILE *f)
 #if !defined(__FreeBSD__) && !defined(__APPLE__) && (defined(__x86_64__) || defined(__ppc64__))
     struct stat64 st;
     if (fstat64(fd, &st) == 0)
-        return st.st_size;
+        return static_cast<size_t>(st.st_size);
 #else // unix 32 bits or osx
     struct stat st;
     if (fstat(fd, &st) == 0)
-        return st.st_size;
+        return static_cast<size_t>(st.st_size);
 #endif
 #endif
     throw spdlog_ex("Failed getting file size from fd", errno);