Make an implicit cast from int --> uint32_t explicit.

Perhaps this casting should not happen to begin with, but better
to make it explicit where it is happening for readability.  This
fixes a compiler warning.

include/spdlog/common.h

@@ -193,7 +193,7 @@ struct source_loc
     }
     SPDLOG_CONSTEXPR source_loc(const char *filename, int line)
         : filename(filename)
-        , line(line)
+        , line(static_cast<uint32_t>(line))
     {
     }