kconfig/gconf: silent missing prototype warnings

As the `gconf' frontend is un-maintained, go the easy way by silencing
the "warning: no previous prototype for '<fn>'" warnings.
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

scripts/kconfig/Makefile

@@ -227,7 +227,8 @@ HOSTLOADLIBES_qconf	= $(KC_QT_LIBS)
 HOSTCXXFLAGS_qconf.o	= $(KC_QT_CFLAGS)
 
 HOSTLOADLIBES_gconf	= `pkg-config --libs gtk+-2.0 gmodule-2.0 libglade-2.0`
-HOSTCFLAGS_gconf.o	= `pkg-config --cflags gtk+-2.0 gmodule-2.0 libglade-2.0`
+HOSTCFLAGS_gconf.o	= `pkg-config --cflags gtk+-2.0 gmodule-2.0 libglade-2.0` \
+                          -Wno-missing-prototypes
 
 HOSTLOADLIBES_mconf   = $(shell $(CONFIG_SHELL) $(check-lxdialog) -ldflags $(HOSTCC))