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/amd/Kconfig

@@ -44,7 +44,7 @@ config CPU_AMD_AGESA_OPENSOURCE
 endchoice
 
 if CPU_AMD_AGESA_BINARY_PI
-source src/vendorcode/amd/pi/Kconfig
+source "src/vendorcode/amd/pi/Kconfig"
 endif
 
 config AGESA_EXTRA_TIMESTAMPS