module api: add API_EXPORT to key_def functions

It is the rule of thumb to use API_EXPORT with module API functions.

To be honest, I don't see strict reason to use this macro except for
unification with other module API functions.

It adds 'extern', which is default for functions.

It adds __attribute__((visibility("default"))), but symbols to be
exported are listed in extra/exports or src/export.h (depending of
tarantool version, see [1]).

It adds __attribute__((nothrow)), which maybe allows a compiler to
produce more optimized code and also catch an obvious problem and emit a
warning. I don't know.

So the reason for me is unification.

Part of #5273

[1]: 2.5.0-42-g03790ac55 ('cmake: remove dynamic-list linker option')
[2]: 1.6.8-71-g55605c5c9 ('Add __attribute__((nothrow)) to API_EXPORT macro')

src/box/key_def.h

@@ -302,7 +302,7 @@ typedef struct key_def box_key_def_t;
  * \param part_count the number of key fields
  * \returns a new key definition object
  */
-box_key_def_t *
+API_EXPORT box_key_def_t *
 box_key_def_new(uint32_t *fields, uint32_t *types, uint32_t part_count);
 
 /**
@@ -310,7 +310,7 @@ box_key_def_new(uint32_t *fields, uint32_t *types, uint32_t part_count);
  *
  * \param key_def key definition to delete
  */
-void
+API_EXPORT void
 box_key_def_delete(box_key_def_t *key_def);
 
 /**
@@ -322,7 +322,7 @@ box_key_def_delete(box_key_def_t *key_def);
  * @retval <0 if key_fields(tuple_a) < key_fields(tuple_b)
  * @retval >0 if key_fields(tuple_a) > key_fields(tuple_b)
  */
-int
+API_EXPORT int
 box_tuple_compare(box_tuple_t *tuple_a, box_tuple_t *tuple_b,
 		  box_key_def_t *key_def);
 
@@ -337,7 +337,7 @@ box_tuple_compare(box_tuple_t *tuple_a, box_tuple_t *tuple_b,
  * @retval >0 if key_fields(tuple) > parts(key)
  */
 
-int
+API_EXPORT int
 box_tuple_compare_with_key(box_tuple_t *tuple_a, const char *key_b,
 			   box_key_def_t *key_def);