bump: Start working on the next release
build: Renamed CURLX_ONES file list definition to CURLX_CFILES

Renamed the CURLX_ONES file list definition in order to a) try and be
consistent with other file lists and b) to allow for the addition of
the curlx header files, which will assist with Visual Studio project
files generation rather than hard coding those files.
Makefile.inc: Added curlx headers to assist Visual Studio project generation