[llvm-c-test] Make include-all.c do what its name says it does

The purpose of this file is to make sure that all includes in llvm-c
works when included from a C source file (i.e no C++isms sneaked in).
To do this it must actually include all the include files.

Reviewed By: whitequark
Differential Revision: https://reviews.llvm.org/D61567

llvm-svn: 360033