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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360033 91177308-0d34-0410-b5e6-96231b3b80d8