util/kconfig: Fix gconfig build

This linker error was the problem:

build/util/kconfig/zconf.tab.o: In function `conf_read_simple':
/home/jn/dev/coreboot/util/kconfig/confdata.c:413: undefined reference to `kconfig_warnings'
/home/jn/dev/coreboot/util/kconfig/confdata.c:413: undefined reference to `kconfig_warnings'
build/util/kconfig/zconf.tab.o: In function `sym_calc_value':
/home/jn/dev/coreboot/util/kconfig/symbol.c:388: undefined reference to `kconfig_warnings'
/home/jn/dev/coreboot/util/kconfig/symbol.c:388: undefined reference to `kconfig_warnings'
collect2: error: ld returned 1 exit status
/home/jn/dev/coreboot/util/kconfig/Makefile:339: recipe for target 'build/util/kconfig/gconf' failed
make: *** [build/util/kconfig/gconf] Error 1

Change-Id: I4a667c7c15b35618fb9ad536f2be5044b8031ab4
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15505
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/kconfig/gconf.c

@@ -26,6 +26,8 @@
 
 //#define DEBUG
 
+int kconfig_warnings = 0;
+
 enum {
 	SINGLE_VIEW, SPLIT_VIEW, FULL_VIEW
 };