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, but they are not related to asm.js (if I'm right):
```
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
```