rename const power 10

In windows Cygwin 32, the compiling fails and reports:

```bash
modp_numtoa.c:22:21: error: ‘pow10’ redeclared as different kind of symbol
 static const double pow10[] = {1, 10, 100, 1000, 10000, 100000, 1000000,
                     ^
/usr/lib/R/etc/Makeconf:123: recipe for target 'modp_numtoa.o' failed
make: *** [modp_numtoa.o] Error 1
ERROR: compilation failed for package ‘jsonlite’
* removing ‘/usr/lib/R/site-library/jsonlite’
```

Rename the const array will avoid such error.