Some tests use strcmp() function, but do not include the string.h header directly. As a result, such tests do not compile under some conformant implementations. This patch adds the mentioned header file to such tests.
Why? Please, provide reasoning in the context of this very test. My reasoning, for example, is: in standard <string.h> defines strcmp(), <cstring> defines std::strcmp(), this very test uses the first variant of the function call.
For complete consistency with this request, two more tests were patched. Calls to strcmp() replaced with std::strcmp() and <cstring> header added according to the requirement of the standard. The test std/depr/depr.c.headers/string_h.pass.cpp remains untouched since it checks the requirements to <string.h> header.