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

external/libgltf/UnpackedTarball_libgltf.mk

@@ -27,6 +27,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,libgltf,\
 	external/libgltf/patches/get_bitmap_new_syntax.patch \
 	external/libgltf/patches/remove_extra_include.patch \
 	external/libgltf/patches/fmod_error_handling.patch \
+	external/libgltf/patches/extern-C.patch \
 ))
 
 # vim: set noet sw=4 ts=4:

external/libgltf/patches/extern-C.patch

+--- libgltf/inc/libgltf.h
++++ libgltf/inc/libgltf.h
+@@ -48,7 +48,6 @@
+ void gltf_get_camera_pos(glm::vec3 *pos,glm::vec3 *view,glm::vec3 *up);
+ 
+ /*get model center position information*/
+-extern "C"
+ glm::vec3 gltf_get_model_center_pos();
+ /*get camera position information*/
+ extern "C"