change errno.h to cerrno

The former is deprecated by C++14. It's also functionally the same.

From the standard:

19.4
The header<cerrno>is described in Table 43. Its contents are the same as
the POSIX header<errno.h>,except that errno shall be defined as a macro.
[Note: The intent is to remain in close alignment with the POSIX
standard.] A separate errno value shall be provided for each thread.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -49,10 +49,9 @@
 #include "fs/io/GzipOutputStream.hxx"
 #endif
 
+#include <cerrno>
 #include <memory>
 
-#include <errno.h>
-
 static constexpr Domain simple_db_domain("simple_db");
 
 inline SimpleDatabase::SimpleDatabase(const ConfigBlock &block)