AGESA stub 00730F01: Add config.h and kconfig.h to Makefile.inc

The static library builder for the stub that interfaces to the
AGESA binary does not include config.h and kconfig.h, so any
header file changes that depend on Kconfig variables fail.  Force
these two system headers to be included in the build of any AGESA
stub files.

Signed-off-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
Change-Id: I2e8d38fa5aa21cc31b995ee3abe68ab3c3c55a68
Reviewed-on: http://review.coreboot.org/6979
Reviewed-by: Martin Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins)