kbuild: Fix bug in make deb-pkg when using seperate source and output directories

From: Ryan Anderson <ryan@michonline.com>

When running "make O=something deb-pkg", I get a failure that claims I
haven't configured my kernel (I have).  Running it a second time tells
me to run "make mrproper"  (include/linux/version.h got built on the
first run)

Original patch from:
From: Ajay Patel <patela@gmail.com>

With modifications from:
Signed-off-By: Ryan Anderson <ryan@michonline.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/package/builddeb

@@ -41,7 +41,7 @@ else
 fi
 
 if grep -q '^CONFIG_MODULES=y' .config ; then
-	INSTALL_MOD_PATH="$tmpdir" make modules_install
+	INSTALL_MOD_PATH="$tmpdir" make KBUILD_SRC= modules_install
 	if [ "$ARCH" == "um" ] ; then
 		mv "$tmpdir/lib/modules/$version"/* "$tmpdir/usr/lib/uml/modules/$version/"
 		rmdir "$tmpdir/lib/modules/$version"