Fix compiler error with avr-gcc 4.7 and the ntpTest example.

With avr-gcc 4.7 (the version used by installing arduino as a package on debian/ubuntu) the ntpTest example fails to compile with errors such as:

ntpTest.ino:78:22: error: variable ‘janStr’ must be const in order to be put into read-only section by means of ‘__attribute__((progmem))’

This hasn't been seen before because the default version of avr-gcc used by arduino is 4.3 and doesn't catch this issue.  This fix to the example code resolves the compilation problem with avr-gcc 4.7.

The commit was tested by running the updated ntpTest example with both avr-gcc 4.3 and 4.7 versions of arduino and verifying it ran as expected in both cases.