Kconfig: Add missing quotes around source file

All errors are generated by ./tools/qconfig.py -b -j8 -i whatever.
Error look like this:
drivers/crypto/Kconfig:9: warning: style: quotes recommended around
'drivers/crypto/nuvoton/Kconfig' in 'source drivers/crypto/nuvoton/Kconfig'

Signed-off-by: Michal Simek <michal.simek@amd.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 8eeb99e..45c2063 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -398,7 +398,7 @@
 	  for testing that EFI is working at a basic level, and for bringing
 	  up EFI support on a new architecture.
 
-source lib/efi_selftest/Kconfig
+source "lib/efi_selftest/Kconfig"
 endif
 
 config CMD_BOOTMENU