kconfig: fix an "implicit declaration of function" warning

strncasecmp() & strcasecmp() functions are declared in strings.h, not
string.h. On most environments the former is implicitly included by
the latter but on some setups, building menuconfig results in the
following warning:

  HOSTCC  scripts/kconfig/mconf.o
scripts/kconfig/mconf.c: In function ‘search_conf’:
scripts/kconfig/mconf.c:423:6: warning: implicit declaration of function ‘strncasecmp’ [-Wimplicit-function-declaration]
  if (strncasecmp(dialog_input_result, CONFIG_, strlen(CONFIG_)) == 0)
      ^~~~~~~~~~~
scripts/kconfig/mconf.c: In function ‘main’:
scripts/kconfig/mconf.c:1021:8: warning: implicit declaration of function ‘strcasecmp’ [-Wimplicit-function-declaration]
   if (!strcasecmp(mode, "single_menu"))
        ^~~~~~~~~~

Fix it by explicitly including strings.h.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/gconf.c

@@ -18,6 +18,7 @@
 
 #include <stdio.h>
 #include <string.h>
+#include <strings.h>
 #include <unistd.h>
 #include <time.h>
 

scripts/kconfig/mconf.c

@@ -15,6 +15,7 @@
 #include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <signal.h>
 #include <unistd.h>
 

scripts/kconfig/nconf.c

@@ -8,6 +8,7 @@
 #define _GNU_SOURCE
 #endif
 #include <string.h>
+#include <strings.h>
 #include <stdlib.h>
 
 #include "lkc.h"