Re-enabling PACK_STRUCT for MDL files.

Re-enabling PACK_STRUCT for MDL files.

When using this compiler:

    Apple LLVM version 7.3.0 (clang-703.0.31)
    Target: x86_64-apple-darwin15.4.0
    Thread model: posix

The MDL::Frame is aligned in 8-byte boundaries instead of 4. This
generates this error when loading old Quake 1 MDL files:

    ERROR: Failed to load file: Invalid MDL file. The file is too small
    or contains invalid data (File: MDLLoader.cpp Line: 422)

It seems this error was introduced by commit
bd2af6591f20fd8777332d510056a037fbfa0df5 due to compilation warnings.