Fix build when both MUNIT_NO_BUFFER and MUNIT_NO_FORK are defined

[Syeberman]

munit_replace_stderr is used here, surrounded by #if !defined(MUNIT_NO_BUFFER):

munit/munit.c

Line 1426 in fbbdf14

const volatile int orig_stderr = munit_replace_stderr(stderr_buf);

It is also used here, surrounded by #if !defined(MUNIT_NO_FORK):

munit/munit.c

Line 1355 in fbbdf14

orig_stderr = munit_replace_stderr(stderr_buf);

This PR ensures that munit_replace_stderr is available when MUNIT_NO_BUFFER and MUNIT_NO_FORK are both defined.

[Syeberman]

Accidentally created this against my master branch. Opened a new PR on a branch: #103