build: omit source file from autodependencies

When converting a source file from .c to .cc, make will complain that the
old source file was missing.  This is annoying, especially when bisecting
or compile-testing a patch set.

Fix by removing the source file dependency.  Since the dependency is alread
specified explicitly by the makefile, no information is lost.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>