Activate clang_complete for C file sub-types

Since I often write documentation with Doxygen, I automatically set doxygen file sub-type for C files. As a result, vim doesn't activate clang_complete.
This patch fixes problem for doxygen and all others sub-types.
Merge pull request #375 from rasendubi/patch-1

Activate clang_complete for C file sub-types

When writing documentation with Doxygen, some people automatically
set doxygen file sub-type for files. As a result, vim doesn't activate clang_complete.
This patch fixes problem for doxygen and all others sub-types.