kbuild: prevent modpost from looking for a .cmd file for a static library linked into a module

This fixes a compile time warning which occurs whenever a static library
is linked into a kernel module.  MODPOST tries to look for a
".<modulename>.cmd" file to look for its dependencies, but that file
doesn't exist or get generated for static libraries.

This patch prevents modpost from looking for a .cmd file when a module is
linked with a static library

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Ashutosh Naik <ashutosh.naik@gmail.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>