Fix for php_git2_git_diff_delta_to_array

PHP 5.4.9-4ubuntu2.4 
Running on 64-bit Ubuntu Raring

'PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php5/20100525/git2.so' - /usr/lib/php5/20100525/git2.so: undefined symbol: php_git2_git_diff_delta_to_array in Unknown on line 0'

This function seems to be defined in helper.c but it is defined as php_git2_diff_delta_to_array instead of php_git2_git_diff_delta_to_array

Modified diff.c to use the function as defined in helper.c