scripts/nsdeps: get 'make nsdeps' working again

Since commit cdd30ebb ("module: Convert symbol namespace to string
literal"), when MODULE_IMPORT_NS() is missing, 'make nsdeps' inserts
pointless code:

    MODULE_IMPORT_NS("ns");

Here, "ns" is not a namespace, but the variable in the semantic patch.
It must not be quoted. Instead, a string literal must be passed to
Coccinelle.

Fixes: cdd30ebb ("module: Convert symbol namespace to string literal")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>