Fix build when MUNIT_NO_BUFFER is defined and MUNIT_NO_FORK is not

[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 is defined but MUNIT_NO_FORK is not.