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>