build: fix strip_core with in-source build

Yet another fix for building of small library as part of tarantool.
Before this commit slab_arena test fails:

 | [019] Test failed! Result content mismatch:
 | [019] --- small/slab_arena.result	Mon May 20 21:37:46 2019
 | [019] +++ small/slab_arena.reject	Mon May 20 21:47:01 2019
 | [019] @@ -23,3 +23,4 @@
 | [019]  arena->maxalloc = 2000896
 | [019]  arena->used = 0
 | [019]  arena->slab_size = 65536
 | [019] +ERROR: Expected dd flag on VMA address 0x7f3ec2080000

See the corresponding commit in the small submdoule for more info.

small @ 020716d8

-Subproject commit e1e9fe1fd7a696b3f04b1481b5a0b29343372dec
+Subproject commit 020716d8a1337118a2afbfaaf4bd13210e640d5c