Kconfig: comply to Linux 5.3's Kconfig language rules

Kconfig became stricter on what it accepts, so accomodate before
updating to a new release.

Change-Id: I92a9e9bf0d557a7532ba533cd7776c48f2488f91
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37156
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/vendorcode/google/Kconfig

@@ -12,7 +12,7 @@
 ## GNU General Public License for more details.
 ##
 
-source src/vendorcode/google/chromeos/Kconfig
+source "src/vendorcode/google/chromeos/Kconfig"
 
 config GOOGLE_SMBIOS_MAINBOARD_VERSION
 	bool