[1.3.0] Remove mbstring from phalcon_c_priority_files.php

Is it intended for the Generator to have ext/kernel/mbstring.h and ext/kernel/mbstring.c in  phalcon_c_priority_files.php? I believe, no

These results in the following errors:
C:\php-sdk>php F:\SVN\cphalcon\build\gen-build.php

```batch
Generating safe build... 
Fatal error: Uncaught exception 'Exception' with message 'file(F:/SVN/cphalcon/ext/kernel/mbstring.h): failed to open stream: No such file or directory on line 168 in F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\File\PhalconC.php' in F:\SVN\cphalcon\build\_resource\Phalcon\bootstrap.php on line 14

Exception: file(F:/SVN/cphalcon/ext/kernel/mbstring.h): failed to open stream: No such file or directory on line 168 in F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\File\PhalconC.php in F:\SVN\cphalcon\build\_resource\Phalcon\bootstrap.php on line 14

Call Stack:
    1.0131     127760   1. {main}() F:\SVN\cphalcon\build\gen-build.php:0
    1.0361     149160   2. Phalcon\Build\Generator->run() F:\SVN\cphalcon\build\gen-build.php:9
    1.1067     248608   3. Phalcon\Build\Generator_Safe->run() F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator.php:52
    1.1067     249008   4. Phalcon\Build\Generator_Safe->generateFiles() F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\Safe.php:84
    3.7714     284368   5. Phalcon\Build\Generator_File_PhalconC->generate() F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\Safe.php:95
    3.9134     310960   6. Phalcon\Build\Generator_File_PhalconC->addPriorityFiles() F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\File\PhalconC.php:72
    4.7635     319880   7. Phalcon\Build\Generator_File_PhalconC->appendSource() F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\File\PhalconC.php:131
    4.7635     319944   8. file() F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\File\PhalconC.php:168
    4.8025     321264   9. Phalcon\Build\{closure}() F:\SVN\cphalcon\build\_resource\Phalcon\Build\Generator\File\PhalconC.php:168
```
This PR removes mbstring header and source file from phalcon_c_priority_files.php and let Generator pass through