build: include header "unit.h" after c++ headers

Header "unit.h" contains `ok` and `is` macros used to check test cases.
The problem is such simple names can be used in C++ STL library headers
(it's OK because such short names can be hidden in a namespace), so when
including, for example, header "vector" after "unit.h", build can fail
because function declaration or definition in the C++ header will turn
into a macro invocation. I faced this problem building Tarantool on
MacOS with SDK of 14.4 version.

NO_TEST=fix build
NO_CHANGELOG=fix build
NO_DOC=fix build