Fix indentation

Change-Id: If94971484109fb63d2e20af77d023e6552a02ba8
Don't use extern "C" for a function which return C++ types

Fix error: 'gltf_get_model_center_pos' has C-linkage specified, but returns
user-defined type 'glm::vec3' (aka 'tvec3<mediump_float>') which is
incompatible with C.

I don't really understand the reason for the extern "C" use in
libgltf.h. After all, the header clearly is intended to be included from C++
code (after all, the use of 'extern "C"' is unconditional and it is not valid
in C), and the implementation of the functions is in C++. Also, we build
libgltf as a static library, so it can't be the case that we would need to
look up its symbols dynamically (when unmangled names would be better). But
maybe I am missing something.

Change-Id: I19f025610301f8c535178a83f4ab2e58455bad57