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/system/Error.hxx

@@ -92,7 +92,8 @@ FormatLastError(const char *fmt, Args&&... args) noexcept
 
 #endif /* _WIN32 */
 
-#include <errno.h>
+#include <cerrno>
+
 #include <string.h>
 
 /**