build: fix compilation against tarantool 2.10.3+

An inclusion of `string.h` header file was removed from
`tarantool/module.h` in tarantool 2.10.3, see [1].

This module uses `strstr()`, `strlen()` and `memcpy()` functions defined
by `string.h`, so it must include this header.

The problem affects the `Debug` build type, which is default for the
`cmake . && make` build flow (it is usually used by developers of the
module). This build type adds `-Werror` compilation flag. However the
problem doesn't affect the `tarantoolctl rocks install tuple-keydef
[<version>]` build flow, because it uses the `RelWithDebInfo` build
type. All compiler warnings are just warnings here.

[1]: tarantool/tarantool#7663

Fixes #24

Author: Totktonada

tuple/keydef.c

@@ -31,6 +31,7 @@
 
 #include <assert.h>
 #include <stdint.h>
+#include <string.h>
 #include <lua.h>
 #include <lauxlib.h>
 #include <tarantool/module.h>