logger: fix 'Undefined symbols for architecture x86_64' on clang for _min_write_chunk

src/modules/logger/log_writer.cpp

@@ -8,6 +8,8 @@ namespace px4
 {
 namespace logger
 {
+constexpr size_t LogWriter::_min_write_chunk;
+
 
 LogWriter::LogWriter(size_t buffer_size) :
 	_buffer_size(math::max(buffer_size, _min_write_chunk))

src/modules/logger/log_writer.h

@@ -63,7 +63,7 @@ private:
 	}
 
 	/* 512 didn't seem to work properly, 4096 should match the FAT cluster size */
-	static const size_t	_min_write_chunk = 4096;
+	static constexpr size_t	_min_write_chunk = 4096;
 
 	char		_filename[64];
 	int			_fd;