cmake: use extra compiler warnings only on gcc6+

cmake: use extra compiler warnings only on gcc6+

gcc version 4.8.5 20150623 (Red Hat 4.8.5-39) and -Wextra produces
following:

 fwtool.c:288:9: error: missing initializer for field 'cur' of 'struct data_buf' [-Werror=missing-field-initializers]
  struct data_buf dbuf = {};
         ^
 fwtool.c:37:8: note: 'cur' declared here
  char *cur;
        ^

Ref: https://github.com/openwrt/openwrt/commit/4ba8f7b1ef1e4c0607185a41c06b51928c625d8b#commitcomment-35906526
Ref: http://lists.infradead.org/pipermail/openwrt-devel/2019-November/020039.html
Signed-off-by: Petr Štetiar <ynezz@true.cz>