Removed some freetype ASM_JS warnings

Hi,
This commit removes some function pointer cast warnings when compiling freetype with asm.js, as discussed in the mailing list. Some warning remain though:
```
WARNING: While resolving call to function 'FTC_GNode_Compare' arguments were dropped!
WARNING: While resolving call to function 'FTC_GNode_Compare' arguments were dropped!
Warning: cannot have duplicate variable names: $strongs_0_i
```