makefile: merge GENERATED_HEADERS & GENERATED_SOURCES variables

makefile: merge GENERATED_HEADERS & GENERATED_SOURCES variables

The only functional difference between the GENERATED_HEADERS
and GENERATED_SOURCES variables is that 'Makefile' has a
dependancy on GENERATED_HEADERS, causing generated header files
to be created immediatey at the start of the build process.
There is no reason why this early creation should be restricted
to the .h files, and not include .c files too. Merge both of
the variables into a single GENERATED_FILES variable to make
it clear it is for any type of generated file.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170228122901.24520-2-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>