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/FatalError.cxx

@@ -30,7 +30,7 @@
 #ifdef _WIN32
 #include <windows.h>
 #else
-#include <errno.h>
+#include <cerrno>
 #endif
 
 static constexpr Domain fatal_error_domain("fatal_error");