Fix native build for CentOS 7

This fixes the native build error on CentOS 7 dotnet#2778 (comment).

Though the managed build subsequently fails with:

```sh
CoreRT native components successfully built.
~/corert
Installing dotnet cli...
Restoring BuildTools version 1.0.26-prerelease-00821-01...
Failed to initialize CoreCLR, HRESULT: 0x80131500
ERROR: Could not restore build tools correctly. See '/root/corert/init-tools.log' for more details.1
Initializing BuildTools...
/root/corert/buildscripts/../init-tools.sh: line 121: /root/corert/packages/Microsoft.DotNet.BuildTools/1.0.26-prerelease-00821-01/lib/init-tools.sh: No such file or directory
ERROR: An error occured when trying to initialize the tools. Please check '/root/corert/init-tools.log' for more details.1
Using CLI tools version:
1.0.0-preview3-003223
```

src/Native/libunwind/src/DwarfParser.hpp

@@ -12,6 +12,7 @@
 
 #ifndef __DWARF_PARSER_HPP__
 #define __DWARF_PARSER_HPP__
+#define __STDC_FORMAT_MACROS
 
 #include <inttypes.h>
 #include <stdint.h>