lib: Wrapper a function to make gcc-10 happy

lib: Wrapper a function to make gcc-10 happy

gcc-10 is complaining:

lib/frrscript.c:42:14: error: cast between incompatible function types from ‘const char * (*)(lua_State *, const char *)’ to ‘void (*)(lua_State *, const void *)’ [-Werror=cast-function-type]
   42 |   .encoder = (encoder_func)lua_pushstring,
      |              ^

Wrapper it to make it happy.  Not sure what else to do.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>