Update vendorcode/intel/makefile for coming FSPs

Other FSPs have more than just the initial fsphob.c source file.
Add any .c files in the srx directory to the ramstage build.

Change-Id: I5118bdcca44935b579809c4fc9566ab7914a6e4b
Signed-off-by: Martin Roth <martin.roth@se-eng.com>
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/5454
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Aaron Durbin <adurbin@google.com>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>