kbuild: add include linux/kconfig.h in config.h

Allow to use define CONFIG_IS_ENABLED
in include/config_fallbacks.h

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay73@gmail.com>
diff --git a/scripts/Makefile.autoconf b/scripts/Makefile.autoconf
index 2f85eb9..2a967ff 100644
--- a/scripts/Makefile.autoconf
+++ b/scripts/Makefile.autoconf
@@ -110,6 +110,7 @@
 	echo \#include \<config_uncmd_spl.h\>;				\
 	echo \#include \<configs/$(CONFIG_SYS_CONFIG_NAME).h\>;		\
 	echo \#include \<asm/config.h\>;				\
+	echo \#include \<linux/kconfig.h\>;				\
 	echo \#include \<config_fallbacks.h\>;)
 endef