Fix some clang-tidy constness warnings

Author: joto

include/osmium/io/detail/read_write.hpp

@@ -106,9 +106,10 @@ namespace osmium {
                     return 0; // stdin
                 }
 
-                int flags = O_RDONLY;
 #ifdef _WIN32
-                flags |= O_BINARY;
+                const int flags = O_RDONLY | O_BINARY;
+#else
+                const int flags = O_RDONLY;
 #endif
                 const int fd = ::open(filename.c_str(), flags);
                 if (fd < 0) {

include/osmium/osm/timestamp.hpp

@@ -149,7 +149,7 @@ namespace osmium {
 
         void to_iso_str(std::string& s) const {
             std::tm tm; // NOLINT(cppcoreguidelines-pro-type-member-init,hicpp-member-init)
-            std::time_t sse = seconds_since_epoch();
+            const std::time_t sse = seconds_since_epoch();
 #ifndef NDEBUG
             auto result =
 #endif

include/osmium/util/timer.hpp

@@ -79,10 +79,10 @@ namespace osmium {
 
         Timer() = default;
 
-        void start() const noexcept { // NOLINT(readability-convert-member-functions-to-static)
+        void start() noexcept { // NOLINT(readability-convert-member-functions-to-static)
         }
 
-        void stop() const noexcept { // NOLINT(readability-convert-member-functions-to-static)
+        void stop() noexcept { // NOLINT(readability-convert-member-functions-to-static)
         }
 
         int64_t elapsed_microseconds() const noexcept { // NOLINT(readability-convert-member-functions-to-static)

test/data-tests/testdata-xml.cpp

@@ -553,7 +553,7 @@ TEST_CASE("Reading OSM XML 200: Using Reader asking for ways") {
     const osmium::io::Header header{reader.header()};
     REQUIRE(header.get("generator") == "testdata");
 
-    osmium::memory::Buffer buffer = reader.read();
+    const osmium::memory::Buffer buffer = reader.read();
     REQUIRE(0 == buffer.committed());
     REQUIRE_FALSE(buffer);
     reader.close();